The "Wax or Wane" achievement requires you to play a series of games from the same island through the "Second Wave" scenario, but altering the island by either adding boards with "Dream of the Untouched Land" or removing boards with "Cast Down Into the Briny Deep", both major powers, until you've won a game with each spirit count from 1 to 6.

Each game played in that series must include spirits that haven't yet participated, which means you need to play at least a total of 1+2+3+4+5+6 = 21 different spirits accross all games, out of a total of 29 spirits currently implemented as I'm updating this. Different aspects do not count as different spirits for this purpose!

Do note that when you prepare the board for the next round after you've altered it, it will give you the option to restore the island to its original state(also the option to play with an empty board if you grew the island), make sure you pick the option to change the spirit count instead!

This guide assumes you want the easiest time possible which means that you'd play on difficulty 0 but also that you want to stack the state of the board in your favor for the next game.

This means that by the time you get to the end of a game you want to:

• Remove every blight from the island
  
• Have 1 of every kind of token except Strife on the island per spirit in the next game, in places where they will likely have the most value, which means you'll want to give more priority to token-placing powers
  
• Have 8 Dahan per board, ideally never singled out and spread equally throughout land types and in lands with more adjacencies. Note that if you stack more than 2 Dahan in a land you'll get to push them around when setting up for the next game, so you often won't need to manually set your ideal dahan placement during the game
  
• Get a valuable power on every spirit for the next game spirits to inherit and use in invader phase III through the scenario rule

Note that if you didn't flip the blight card, you will take your blight reserve as-is for the next game, regardless of the change in spirit count, I will go into further details about how this affects your strategy in a further section.

My own plan was to remove 1 board every game.

I failed a first attempt at game 3 because of an oversight: I had chosen "Cast Down Into the Briny Deep" as a power to inherit, thinking it would remove the need to find it in the major powers deck. I discovered the hard way something I assumed wrong: Inherited powers do NOT have the threshold effect if said threshold requires more than 10 elements. It's written plainly on the front of the scenario card, so it's purely my fault. I therefore couldn't find the power again to cast it in time and decided to start over instead of having a second 4-spirit game with the spirits I hadn't planned to use.

Another thing that made the attempts harder and that I kept doing into my successful run was to not remove the content I didn't need. Since Cast Down is a Branch & Claw power, the Jagged Earth content is not needed, and if you disable it you'll have an easier time finding Cast Down in the Major Powers deck.

6 Spirits

I went for a caldera layout (see image) to have a good compromise between being able to stretch presence into other boards but not sacrificing the ability to create safe pockets too much. Make sure there's no circling arrow showing when you're done setting it up, otherwise it means the game did not detect that the last board you placed is adjacent to one of the boards around it.

• Ocean, as a potential Cast Down caster and for synergies
  
• Lure, to strike it from the list early since I'm not too good with it and as a complement to Ocean. Also a token generator
  
• Teeth, to strike it from the list early since I think it's rather weak to the later games' dangerous initial board state
  
• Earth, as a potential Cast Down caster, although one that doesn't draw that many powers, so 6 spirit count is the ideal time to get rid of it
  
• Starlight, mostly for blight removal, as a tertiary potential Cast Down caster, for overall safety/consistency and to breeze through the Minor Powers deck to find all the blight removal & support needed
  
• River, for massive Ocean synergy, making more dahans to stack the board in my favor in subsequent games and to provide energy to Ocean or Starlight should one of them find Cast Down

This one went near flawlessly, the sheer amount of Minor Powers drafts made removing blight easy.
I ended with all the blight back on the card for a total of 19, and 3 or 4 boards with the full count of 8 dahan, as well as every type of token on every board, though not in absolutely ideal places

5 Spirits

• Serpent, as the go-to Cast Down caster, fully benefitting from an island clear of all its blight
  
• Green, to accelerate Serpent's dominance and have general safety
  
• Eyes, consistency, because I had to fit him somewhere
  
• Heat, same as Eyes
  
• Fractured, To finish generating the Dahan to 8 per board and I wanted him in a large team with plenty of support in case Cast Down ended in the Days That Never Were, so it'd be easy to use. Also I think it synergizes well with defensive spirits

I ended up putting way too much defense and overall power in that team. Nonetheless, that meant No blight was added. I also completely finished populating the island with Dahan, more than 8 per board in fact but you can only keep 8. Serpent ended up with Cast Down, used it on its own starting board and everything went fine.

4 Spirits

• Mud, as my only Cast Down caster, since it can draft a lot of cards and has the elements for it
  
• Vengeance, I wanted to use him quickly since his early game is much weaker when you don't want to blight at all.
  
• Minds, an overall consistent spirit that needs no Major Power
  
• Thunderspeaker, as an offensive spirit benefitting fully from the maxxed out Dahan Population

I blighted a little on this one but cleaned it up before the end. Cast Down was at the very bottom of the Major deck but Mud ended up cycling through it just enough to cast it in fast with some "Sky Stretches to Shore" on the very last turn. This wouldn't have been an issue had I removed the Jagged Earth content. My biggest mistakes here were not stacking disease on Vengeance's starting board, spreading it instead near the end of the previous game, same thing with beasts on Minds' board, and overall not putting too much power in this team. Swapping Vengeance and Eyes might have been better.

3 Spirits

• Memory, as a Major Power drafter and to help the others hit the threshold should they find Cast Down
  
• Fangs, to go through the Minor Power deck quickly and find all the support needed to enable the thresholded Cast Down play
  
• Whirlwind, as a generally consistent spirit with good early game that isn't bad at Major Powers, had to put him somewhere

Memory + Fangs meant with the element help and overall support, anyone could play Cast Down. I did have to dig it up once again at the very bottom of the deck, and I once again barely had the time, though this wouldn't have been an issue without the JE content

2 Spirits

• Keeper, to go through the Major Deck quickly and as an overall powerhouse
  
• Lightning, to deal with the absurd amount of towns in the initial board states efficiently, to help with Keeper's early game and to ensure only Keeper would draft Major since there's no way Lightning would cast it without massive support

Something extremely funny happened on this one, on turn 2, I drew the "Numinous Crisis" Event:
Option 2 would have damaged my early game too much, and I figured I didn't need my 19 blight pool for the last game, so I converted it into energy, both spirits ending up with about 30 energy each. I then found Cast Down nearly instantly, enabling me to draft Majors on Lightning as well without fear of being unable to threshold it. The invaders nonetheless managed to blight Lightning's board before I sunk it, but it was irrelevant since the blight pool would reset to 3 for my solo spirit game, and said spirit wouldn't ever get to flip the card.

1 Spirit

• Stone, to completely breeze through the last game, being unbothered by how many cities were built, ignoring the fact it left the board all bighted up since it was the last game.

It went about as well as you can expect a Stone game to go against no adversary. The game was short and the achievement was gotten.

Since the condition to meet is to play 1,2,3,4,5 & 6 spirit games in any order, that means there's three ways to approach the problem:

1. The Waning Strategy - Cast Down Into the Briny Deep

This is the one I used. It requires at least 1 spirit able to reliably cast and threshold Cast Down every game except the last.

• +Gives you an absurd margin on the blight card if some of your games are dangerous
  
• +Easy time keeping your board populated with tokens and especially Dahan for the next games
  
• +The last games being the toughest, having less spirits involved means you can use your best spirits then.
  
• +6 and 5 spirits in the first two games means you'll easily draft the blight removing and dahan generating powers you want
  
• +You can remove the Jagged Earth powers to thin out both decks
  
• +One spirit will inherit an extra power each time.
  
• --Requires you to find & threshold the most expensive card in the game every time 5 games in a row, can't inherit it
  
• --Can't draft Major Powers on spirits that won't be able to Cast Down, in case they find it in place of the dedicated spirits.
  
• --Have to split all the potential Cast Down casters across 5 games

2. The Waxing Strategy - Dream of the Untouched Land

The other obvious strategy. It requires using Dream of the Untouched Land 5 times.

• ++You can inherit Dream on following games with the threshold effect, which means you only need to find it & threshold in the first game and 2 other times
  
• +Dream is easier to cast & threshold than Cast Down
  
• +If you fail to threshold Dream early on, you use less backup spirits
  
• +Stacking tokens/dahan & removing blight in the first game can be easier for 1 spirit that is good at it rather than having to spread the job across 6 boards for 6 spirits
  
• +You only need to plan around only 3 spirits casting Dream
  
• +You can remove the Branch & Claw Powers to thin out both decks
  
• -Requires more good spirits for the late tougher waves
  
• -One spirit will not inherit any power each game
  
• --When you make it, your new land is empty of disease and most importantly of Dahan, and if you don't add 2, you won't get more on setup.
  
• --If you don't flip the blight card, you carry a very small blight pool of 4 into the next games that include more and more boards

3. The Hybrid Strategy - Using both Dream and Cast Down

The final option: Using Cast Down several times in the first 6 spirit game so that you can Dream after.
Note that you can't use Dream's threshold several times in the same game, so you can only do it this way, but you can plan to go down to 2, 3 or 4, build back up to 5 and Cast down several times again. You have to start at 6 though, otherwise you'd have to repeat the spirit count of the first game in the process of building up to 6.

• +Same absurd blight pool as the Waning Strategy
  
• +Doesn't require to spread your Cast Down candidates across many games, only at high spirit counts where support Minors are readily available
  
• ++Generally, since waxing and waning present different challenges, they require different strengths from the spirits, allowing you to use your best spirits for each situation instead of diluting those suitable spirits across every step of your plan
  
• +If you need Dream two games in a row, you can just inherit it for the 2nd
  
• -Needs to populate the new board when waxing
  
• -Needs to dig potentially through the whole Major deck with both expansions
  
• --Still need to find,pay and threshold Cast Down, and need the time and/or resources to cast it several times.
  
• -Needs more good spirits for tougher waves than the Waning Strategy

Which Strategy should I choose?
I believe they are all viable, but if you're in doubt it will most likely be easier to plan your teams around the hybrid approach.

Bringer of Dreams and Nightmare
-May easily blight early or with bad drafts
Note: DO NOT USE WHEN WANING, This spirit wants Major Powers but cannot destroy anything, hence should it find cast down you will have to either let it go, empty and reshuffle the deck hoping to find it in time, or take it and exchange it through a thresholded Entwined Power. This is extremely risky either way.

Devouring Teeth Lurk Underfoot
-Overall weak against the initial board state of later waves

Downpour Drenches the World
+Removes some blight natively
+Decent Candidate for Dream

Eyes Watch from the Trees
+Decent candidate for Dream
+The defend boon can really help prevent blight in an emergency

Fathomless Mud of the Swamp
+Great candidate for Cast Down and Decent candidate for Dream through power gain and matching elements
+Gives extra power gain to other potential Cast Down/Dream candidates

Finder of Paths Unseen
+Easily moves extra Dahan into a new board when waxing, as long as someone made plenty of extra ones while you waited to cast Dream
+Decent candidate for either Cast Down or Dream
+Easily transports other spirits' presence, notably so they can populate a new board with tokens
-May blight early

Fractured Days Split the Sky
++Slip the Flow of Time can greatly help accelerate Cast Down or enable the threshold through extra plays
++Can generate a ton of Dahan
+Potential candidate for either Cast Down or Dream depending on Days That Never Were
-Might need some help on its board or a reliable neighbor to Pour Time Sideways into

Grinning Trickster Stirs up Trouble
+Generates some beast
-May blight at any time with its innate
-Doesn't want to remove blight

Heart of the Wildfire
-Leaves blight on its board and typically overall adds some by the end, especially with bad drafts preventing you from getting the elements for bigger cleanups through innate
-Overall weak against the initial board state of later waves

Keeper of the Forbidden Wilds
++Great solo candidate for either Dream or Cast Down thanks to its great drafting capability and end game tracks
+Generates Wilds
+Removes some blight natively
-May blight early

Lightning's Swift Strike
+Easy access to slow as fast for Cast Down or to prevent blight in emergencies
-May blight early
Note on aspects: Wind is fine and Pandemonium is also fine if the board is properly filled with Dahan, Immense is better suited for bigger problems rather than the initial board state of later waves

Lure of the Deep Wilderness
++Generates a bit of every token you want natively
-May blight early, especially with bad drafts

Many Minds Move as One
+Generates beasts
+The boon may prevent a blight in an emergency

Ocean's Hungry Grasp
+Decent Cast Down candidate, though may likely require help on energy at high spirit count
-Will blight inland without lucky drafts

Rising Heat of Stone and Sand
~Nothing noteworthy here, a solid spirit but doesn't bring anything special to plan around

River Surges in Sunlight
++Boon of Vigor enables plenty of Cast Down/ Dream candidates
+River's bounty populates Dahan
Note on aspect: Sunshine is horrible, do not play it. Travel is a small improvement but stick to Core River if you're more comfortable with it

Serpent Slumbering Beneath the Island
++Massive support while stalling its own board
+Great candidate for either Cast Down or Dream, just make sure you draft enough cards

Shadows Flicker Like Flame
-Typically plans to blight early even in low difficulty games
-Overall weak against the initial board state of later waves
Note on aspects: Madness and Foreboding are your best shot at limiting the damage, should you choose to play Shadows

Sharp Fangs Behind the Leaves
+Generates Beasts
+Easily cycles through the Minor deck to find all the support minor powers you might need.

Shifting Memory of Ages
++Can transform anyone with average or better energy generation into a Cast Down/Dream candidate, including himself
-May blight early

Shroud of Silent Mist
-Overall weak against the initial board state of later waves

A Spread of Rampant Green
+Decent Dream candidate
+Gift of Proliferation enables some spirits to become Dream or Cast Down candidates
+Can often afford to go prevent a blight on another board

Starlight Seeks its Form
+Decent candidate for either Dream or Cast Down
+Can specialize into plenty of blight removal
+Easily cycles through the minor deck to find all the support minor powers you might need.

Stone's Unyielding Defiance
+Generates Badlands
+Decent candidate for either Cast Down or Dream, just make sure you draft minors for your elements
+Barely cares about initial board state as long as it starts with no blight
+Can add more blight on the card than it's supposed to have
--Typically leaves plenty of blight for the next game, if its board is still in play
Note: For the last game or on the board you plan to destroy, it's one of the most reliable spirits you can choose

Sun-bright Whirlwind
~Nothing noteworthy here, a solid spirit but doesn't bring anything special to plan around

Thunderspeaker
+Easily moves extra Dahan into a new board when waxing, as long as someone made plenty of extra ones while you waited to cast Dream

Vengeance as a Burning Plague
+Generates Disease
-May blight early
-Weak early when trying not to blight or without lucky drafts

Vital Strength of the Earth
+Removes some blight natively
-Overall weak against the initial board state of later waves
Note on Aspect: Since we want to preserve Dahan and we're not playing against Adversaries adding extra pieces or having extra power, Core is much more appropriate than Resilience. Might is similarly less suited to the small problems.

Volcano Looming High
-Annihilates every Dahan that gets in its way
Note: In a hybrid strategy, you can plan to not add Dahan to a new board then Cast Down that same board next game, in which case Volcano is perfect for that board, since it really doesn't need the Dahan it would kill anyway. It can be a good pick for the last game as well, including the last game of a waxing strategy that didn't include Dahan on its last board