Gedonia is not too hard to get 100% achievement, but it does have some tricky and/or missable achievements. These guide cover my experience during the game, going totally vanilla, with no mods or save edits, and with some help found online and put together here (like the mystery solutions and mount locations).

Make regular saves before entering dungeons, and be warned of some possible spoilers.

Also, feel free to contribute if you have any tips or easier ways than the ones described here. This is a work in progress guide, and can be missing some information, but can always be updated anytime.

Hope you enjoy as well as I did!

Get a level up
Can't miss this one.

Catch any fish
Can be done in the starting village. Get the fishing rod from the npc in the lake and fish there. It's a nice source of money in the beginning to catch and sell the first few fish.

Get a horse mount
Just complete the quests from Mr. Alton in the starting village.

Aquire dog companion
Recruit the dog as a companion in the starting village. You need some charisma stats for this one.

Become a member of any faction
Just join any faction and complete the questline. I Recommend joining mid to late game as the final quests in the chain are quite challenging. Be warned that joining a faction may restrict joining rival factions.

Return sir Richard remains to the coffin
In the dungeon "Cursed Prison" on golden forest, you need to get a ring from a coffin to progress through the level. After killing the final boss, return the ring and the remains looted from the boss to the coffin that you got the ring, near the entrance.

Cursed prison Location in Golden Forest:

Complete main story of the game
Unmissable as you progress the main story.

Climb to the top of the guiding mountain in Endless Fields
You can easily do this with the Flying skill or Flying mount in late game, but it's possible to climb it jumping if you have the patience.

Before entering the Citadel of Evil in Darklands, be well prepared to kill everything inside in the first try, and you can get the 4 achievements. There are several rooms, and the mentioned bosses are grouped together in different rooms. This is the hardest dungeon in gedonia, but should be done before going to northern realms, as there are nice epic loot to be found.

Kill all world bosses on the map
You need to kill and loot every red skull marked boss on the map, including the northern realms world bosses.

Updated as of version 1.46a January, 30, 2024:

To get I am the boss achievement, now you only need to kill bosses in Gedonia, respawnable bosses in Northern Kingdoms are ignored for this achievement

Complete all secondary quests in the world
Complete all quests marked with a yellow exclamation (!) mark on the map, including northern realms quests.

Explore the whole world map
Just for the gedonia main map. There are some areas that seems to remain undiscovered, but revealing the obvious fog areas should do the trick. After getting the flying mount and doing the side quests this should be easy.

Get all mounts in the game
There is a very helpfull guide in the Gedonia Wiki page, with mounts and companions location. So check it out: https://gedonia.fandom.com/wiki/Mounts_%26_Companions

Solve all mysteries on the map
There is also a great guide from StormHorse on youtube for every mystery, marked with an interrogation (?) mark on the map. If you need help with any of them, check the video for each region:

Uncover your hidden talents
On the left side of the entrance to the temple of living weapons in northern realms, there is a quest to pass a trial. Kill the boss and it will unlock a new passive ability tree that gives very helpful bonus stats. The boss fight is in a small area, so keep running if you're a mage.

Beat atleast 5 waves in the Frozen Temple
This is an endless dungeon in the Northern Realms region that you keep the rewards after dying, so, try it until you can beat 5 waves in a run. It seems this is one of the best places to farm living weapon xp and marks of ascendancy for other 2 achievements.

Evolve any living weapon to the highest stage
To get a living weapon, you need to kill a world boss or high end enemy in Northen Realms and get the "Ancient husk of a living weapon" item to craft a living weapon in the Temple of Living Weapons.

After that, you need to get living weapon xp by killing mobs, collecting "Frost energy" from world map events and obelisks, or doing the frozen temple. Frozen temple seems to be one of the fastest ways to get weapon xp, but it shares the world events respawn timer. So, do one, do something else (frozen caves, world events, expeditions) and return later when all the world events have respawned.

After leveling up the weapon two times (5k xp for stage 2, and 10k xp for stage 3), the achievement should pop.

Summon a rain of fish
This is a cooking recipe. Just gather the fish (8 sturgeon, 4 tuna, 2 pike) craft and use the item for the achievement. Remember that you can always respec with oblivion potion if you don`t want cooking to be part of your build.

Feed 5 penguins in a row
In Northen Realms feed 5 raw fish to penguins. They are located west and nortwest on the map, in small islands. I gave five fish for three penguins, so, probably you don`t need different penguins to count.

Ascend any armor to the highest level
To ascend armor, you will need to be at least lvl 45 and do the expeditions quest.
With that done, you must gather ascension gems to upgrade an epic armor.

3 broken shards of ascension = 1 mark of ascendancy (lvl 1 upgrade)
10 marks of ascendancy = 1 elaborate crystall of ascendancy (lvl 2 upgrade)
25 elaborate crystall of ascendancy = 1 sublime orb of ascendancy (final upgrade)
Upgrading to final tier, the achievement should trigger.

The expeditions on gedonia map awards broken shards, but it's only doable for when you are starting to get to the endgame (for some loot), as it becomes very grindy.

After getting your first upgrades and completing the living weapon for the other achievement, you can start trading "Husk of a living weapon" for "Elaborate crystal of ascendancy" in front of the temple of living weapons. 1 husk = 3 marks, 5 husks = 2 Elaborate crystals. You should just trade when you have 5 husks, better outcome.

There are trials in the northern realms to get marks and husks of living weapons. These trials can be found inside almost every frozen cave in the northern realms, frozen temple, and around the map, like in Erynar and the Fire energy world event. These are much more harder than expeditions but much more rewarding, so be prepared.

Summon a magmatic fish spirit
This is a cooking recipe. It needs max level of cooking skill, 3 magmatic birdfish, 5 concentrated fire rage and 8 fire essence to craft the item.

The fire itens can be obtained by killing fire mobs and elementals.

The magmatic birdfish is the hardest one: get it by fishing in the pools of lava on Darklands region.

To get the fish faster, I recommend using worms, fishing rule, and getting the special fishing pole from the fishers village near main town, otherwise, it will take too much time for the fish to bite, as there is no special spots to fish on lava pools.

Defeat whole coven of demons at the same time
You need to complete a quest located northeast in Northern Realms (in the mountains, left of the fire essence world event) without weakening the demons. Skip the optional objetive and go directly to the northern objective to disable the blood source of demons.

That's possible one of the hardest fights in the entire game. It's better to clear whatever is left, gear up, get levels, and do this after every other quest is complete to have a better chance.

I've done it with a max lvl lightning mage, using the ultimate ability to get invulnerable while dealing damage, and running around while the ultimate was in cooldown.