Played 1800+ hours, if you care about that. (Note: For reasons, I can't currently reply to people)
I tried to keep this as concise as humanly possible without leaving anything important out.

I tried to limit myself to just what I could reasonably expect someone to have (no artifact traits were used here or anything). Only used starter spells. Didn't utilize extra artifact slots super heavily except on carries.

This is focused primarily on the story, with not much note about the late game strats aside from an example build, with the presumption being that you can read the traits yourself and at least somewhat grasp why they're good and what you should maybe take for your own build.

To reiterate the description, the OTHER GUIDE is OUTDATED and should NOT be followed as you'll run into various issues as a result of that, and it isn't even as fast as the example build I have listed here (which wins most battles in like 5 seconds). If you have questions then using the comment section is fine but the Siralim Ultimate Discord you can find in Steam Forums is preferred.

The first 3 bosses are VERY simple to beat if you accept the grind.

90% of your problems can be solved by overleveling your creatures and targeting down the problem creature(s). Namely, bosses. If you fail to beat a boss during the story, there's a good chance that being a higher level would have fixed that (or sometimes it's dumb luck). Having a high base value for the damage stat you're using is important if you want it to be fast.

PERKS TO FOCUS ON for DAMAGE:
-Executioner until you've BEATEN MYTRLE, then RESET at wardrobe for Magma Diver
-Magma Diver
-Massacre

ALSO, PROTECTION:
-Bolster

Very good perk for tanking damage. If you want it to REALLY put in work then increasing your maximum health as much as possible will make you nearly immortal.

Important things to remember for projects:
-ALWAYS go for the +maximum projects first so you can have more active at once; there is NO DOWNSIDE to having many projects active at once. They don't distribute between them or anything, they just ALL get the full amount of whatever it is that you're farming for them.
-In case you weren't aware, Arcane dust comes exclusively from completing realm quests & creature parts from battling enemies (the ones that walk around), and you can't boost the amount in any way shape or form for EITHER of them, regardless of any loot multipliers etc because project materials are NOT real loot.
-ALWAYS keep upgrading your blacksmith first and foremost, THEN enchanter

Depth 15+ you'll still be mostly doing the same thing, but you'll be able to fuse things and have more creatures appearing for each race in realms that you can go get. Should be noted that at depth 43+ you'll have access to creature leveling (level your creatures up to your highest-level creature's level), so there will still be grind in terms of setting up your team to counter bosses if you need certain traits to do it.

At this point countering the bosses may start to hold a bit more value, but it should be noted that you can grind Venedon's realm (Caustic Reactor) for some very useful spells (such as greater dispel to remove all enemy buffs and ally debuffs) from loot. Spells are a HUGE part of the game and can really help you with a LOT of things. Even attack-focused builds still have spells in case they need them (with the goal being you normally don't, at least not for damage).

PERKS TO FOCUS ON:
-Kindling
-Reckless Abandon
-Inner Focus

DEPTH 31+:
-Asura Gutslasher can be found in Torun's realm (Cutthroat Jungle) randomly from interacting with the little tiki totems; this creature will be a MASSIVE step up as it will let you replace your lead creature with a strong attacker. Reckless Abandon is key, as it will cause the firestorm cast by the kindling perk to critically hit and thus trigger sidewinder, which will cause splashing to all enemies from massacre, thus consistently wiping waves of enemies out as long as the spell deals damage (and thus can crit) and your attack is high enough. THIS is where the autoclear really starts.

The only stats that matter on the lead creature at this point are ATTACK (naturally), and to a lesser extent INTELLIGENCE (just because it will help you get past enemy defense with the spellcast in order to start swinging). Your other creatures are just there to support this creature in normals, and they can deal with any holes in your strategy when you encounter bosses.

Depth 43+ you'll have access to leveling, which will make a big impact, but depth 45 is more important due to unlocking more creatures again (every 15 depth unlocks another creature for each race until you have them all).

PERKS TO FOCUS ON:
-Dreadnaught (dodges might be getting annoying by now)
-Indomitable
-Damage & the rest (not Pyromania though, that's too much spam). Yep that's it, you don't really need any other perks in particular at this point as you've got the important ones already.

DEPTH 45+:
Efllorescent Ent will be a great help if you need better protection against enemy damage, paired with ebony ent which you've had access to from the start.
Centaur Raider will help cleanup quite a bit, since one kill can become 6 just like that.

DEPTH 58+:
Doomguards are fantastic, Diabolic Horde are an option. Chaos Guard (Vigor) can help with tanking better if that's still an issue.
DEPTH 61+:
Unguided from Sanctum Umbra's god shop (Perdition) has 2 more great damage reduction options (one of the most useful in the game), made to be used together (Agnostic & Sadist).

I decided to play through using my own guidelines just in case I missed something, so here are the details on how it went.

NOTE: You can turn difficulty down to (0) in options > gameplay in the menu. There's no benefit to playing on higher difficulty or anything (no, you don't get more levels for beating higher level enemies on difficulty 9 or anything like that).

(1-3): Summoned bard, low-INT personality threw it out & resummoned; leveled to 4 and focused Kiichi down.
(4-6): Had only 3 creatures, levels went up faster as a result, grinded to level 9 and slapped magic missile on the bard to pew pew Myrtle with.

(7-9): Bard default strat was set to cast since Myrtle; targeted down the salamanders every time on my way to him. Summoned Ebony Ent after leveling to 11, as well as salamander & maniac just as fodder allies (Caustic Reactor is crazy) Had Magma Diver and spare points were always in Executioner. Lost the first time, then remembered to put in my perk points and sniped Loid properly (and got lucky with my bard dying and casting due to the fire salamander, CRITTING Loid). Almost decided to just level to 12 (& make fodder levels higher so that they had damage too). Death class creatures like Terror Hound deal 2x damage to chaos by the way...
NOTE: This may have gone more smoothly if I had farmed for an ATTACK-UP personality on a mummy and had been using that as my main attacker instead...

(10-12): Magma Diver And Massacre now both active. Clears going a bit faster now. Summoned a Smog in light of the mummy vs loid epiphany, it had a +ATK -SPD personality. Randomly wiped on realm 11 to some nonsense. Fused clutcher onto unicorn (Unicorn was main parent since it had a +SPD personality, but could've farmed for a perfect one on clutcher). Wiped before the fusion, then did some grinding for the project completion (blacksmith rank 2), did Giran again before finishing and just won from massacre-wiping his team with my now-level-15 asura & level 11 Vivifier/Clutcher fusion (only fusion at the time and kinda didn't help here ngl); 2nd highest level was bard at 14, then 12 for salamander & tree).

-(It's been nearly an hour at this point; could have been faster but I refused to use my brain like when I didn't go for a mummy the moment I set foot in Azural's realm)

(13-15): Completed Blacksmith rank 2 project, maxed artifacts again immediately (no traits on them yet). Equip a bunch of spells I happened to have, summoned and fused a terra vulpes onto salamander as the 2nd parent (life counters death). With a bit of actual thought I decided to play a bit defensive with molten armor casts on the carry; worked out. Started +maximum projects.
(16-18): The clutcher unicorn actually helped a lot here. Went down easy, with luck (was only overleveled by 1 on my carries).
(19-21): 3 big chaos attackers vs sorcery creatures? SO fast and easy. Don't forget perk points...
(22-24): Had reckless Abandon now. Nothing else to note, was fast & easy.
(25-27): Had Kindling & Enduring Rage on now. Realm was a non-issue. Wished for glory for unicorn mana, didn't summon. Abused Molten Armor, carry level 29, the rest avg 27. Wiped a couple of times because of annoying resurrection spells the enemies randomly had (there are a LOT of life-type resurrection spells). About 3 casts seemed to let me one-shot Qila with the attack carry.

~(about another hour had passed by now; Vlora & more so Qila wasted a bit of time...)

(28-30): Realm was a non-issue. Bolster now on (Vitja is a jerk and randomly wiped me because of a Morph: Death her team cast when I would normally COUNTER her; AKA 75% damage reduction vs my carries...). Wiped her the 2nd time thanks to bolster keeping me alive (barely). Remembered I had +maximum projects available since a number of realms ago and started it.
(31-33): Fused Asura Gutslasher onto Smog, now wiping pretty quick and consistently for the most part. Inner Focus now maxed, wiped the boss nearly Turn 0 on the 2nd try (well the boss himself I actually DID kill immediately, he just had a couple allies survive the critical-kindling wombo combo spam).
(34-36): Went smooth, nothing to note.
(37-39): Molten armor on the starter asura (still unfused and high attack) while swinging & targeting Ugat first. Won 1st try.
(40-42): Finished +max projects #2, started enchanter rank 2 & gambling dwarves. Won against him after a couple of wipes with the help of good RNG on sidewinder (took 3 turns of attack spam from the lead creature to down Tellur, and thankfully they had no resurrection). Watch out for sparktails.
(43-45): Went back to blood grove to get efflorescent ent and Centaur Raider, because bolster isn't 100% reliable when one-shots are easily possible and the game's math is goofy. Didn't end up using centaur yet.

~(3rd hour now, Tellur made things rather slow)

(46-48): Suddenly realize I have spare perk points and that I forgot to get Indomitable. Eh, oh well. Dreadnaught & deadly calm taken just because I can. Got the nixes, though not without pain (silenced really slowed things). Got blacksmith rank 3 & enchanter rank 2, start +max projects & blacksmith rank 4. Max artifacts again (rank 30 now; sword, sword, boots, staff). Fused starter asura onto Attack-up speed-down personality centaur, re-equip artifact, remove bard from build for another centaur raider (+INT -SPD but whatever). Auto-wiped boss, but almost lost.
(49-51): Executioner maxed and not changing it since I've got everything, Death Sentence next focus. Auto-wiped boss, but it has rez stuff so had to kill it a few more times and use constriction to keep him from attacking while I'm waiting to reach my carry's turn, just to be safe since he can copy centaur raider and wipe the team (had it on 2nd centaur raider & the tree).
(52-54): Finished dwarves project, placed one in castle. Started enchanter rank 3. Enemy levels & stats starting to get a bit high so to nuke the boss decided to farm for blacksmith project materials in The Barrens (Yseros/snake lady realm), and for god favor for sphinx trait materials. "Enchanted coal" is a random drop from each realm's breakable object btw (for The Barrens, it's a pile of white rocks). After getting helm on tree with pharaoh's boon (elder mane; favor rank 7 with Yseros), won at level 65 (molten armor on a centaur did it).
(55-57): Finished enchanter rank 3, start unlocking pilgrimage missions (project). Finished +max projects, start assassination mission unlock & divination candle. Wiped team of the boss and defended until he died to story damage (just survive a while to kill him). Writhelings are spooky.

~(hour 4 has passed at this point)

(58-60): Summoned a chaos guard (Vigor gives 25% damage reduction), didn't add to team. Auto-wiped boss. Nothing else to note.
(61-63): Wrecked boss almost without issue (good thing I had tank power; didn't even add vigor, still, but if I needed more tank at this point I'd farm Sanctum Umbra for favor to unlock the damage reduction unguideds).

~(At nearly the 4.5 hour mark, the story is done; I used ZERO artifact traits)

Depth 64+:
-Start +max projects (final one).
-Start unlocking bonus realms - Amalgam gardens, Astral Gallery, Damarel, etc, in that order (ABC order; yes, that's literally the best order because divine form is incredible and is rank 12 in amalgam gardens, animations in astral gallery are great, Fae Lands have Asclepias Fae + Bile Slime for a great resurrection combo).
-Start unlocking Reliquary, Arena, Goblet, Refinery, & all other specs, in that order.
-Start unlocking the other missions last (they're not important).
-The plan begins...

At this point you should start focusing on getting favor in these god shops (pilgrimage missions will help a LOT) for their wonderful shop creatures/traits:
-Perdition (Sanctum Umbra)
-T'Mere M'rgo (Amalgam Gardens)
-Yseros (The Barrens; sphinx stats can be useful & flexible)
-Tartarith (Torture Chamber; Mauler Fiend's 'Eyesore' specifically)
-Azural (Frostbite Caverns; best unlock is at favor 35...)
-Alexandria (Damarel; not high-priority, but forsakens can be useful for big carry power)

Additionally, 4080's shop has some useful spells in it. Warp Reality is a useful one. For normal spells, however, you'll wanna just grind in general to unlock all the stuff that you can for your builds if you haven't done any crazy spell grinding yet. RI5 isn't recommended just yet, but may become an option depending on how fast you get your build set up and really rolling hard (wait until after you have a resurrection engine, at least, like Asclepias fae + Bile Slime's trait).

If you ever happen to get the "Imposter Syndrome" (broken mirror) trait, which is exclusively from random loot and only for artifacts, then prioritize getting Mauler Fiend as a 2nd parent onto an Asura Gutslasher (sidewinder) and put that on its sword artifact. The attack value it starts with will be INSANE.

Ideally, you'd eventually want a build that's something like this:
https://berated-bert.github.io/siralim-planner/?b=3d043c8852cf2128613ee74900a092670bf28d0bf700a0921833df30602900a09274f06f4621c18dddcf09fb7935120351acd3485711&s=HK&r=ge_____
(^ Copy and paste it manually into the URL if something isn't showing; Steam tends to remove things from planner links)

Asura Gutslasher / Mauler Fiend / Imposter Syndrome
^ pure-attack sword, %damage reduction, %attack damage
^ Axe of Gonfurian (relic)
Nature Shapeshifter / Pit Guard / Phase Knight (Endurance Aura)
^ pure-health helmet, %damage reduction, [2nd trick slot not important]
Immortal King / Pit Guard / Sphinx Zealot (Pharaoh's Bane)
^ pure-health helmet, %damage reduction, [2nd trick slot not important]
Ebony Ent / Pit Guard / Efflorescent Ent (Black Lotus)
^ pure-health helmet, %damage reduction, [2nd trick slot not important]
Unguided Sadist / Chaos Guard / Unguided Agnostic (Dispossession)
^ pure-health helmet, %damage reduction, [2nd trick slot not important]
Asclepias Fae / Sunclaw Gargoyle / Bile Slime (Secret Recipe)
^ pure-health helmet, %damage reduction, [2nd trick slot not important]

SPELLS: Molten Armor in case you need fast attack scaling for some bizarre reason. And then just various spells with various effects, with an enchantment for "potency based on attack" on them if they mention "small", "moderate", "large" etc in their descriptions. Astral Dimension is important utility in case of spell sealing. Fermented hops from the Gambling Dwarves shop is an incredible anti-boss spell (you can't cast booze spells manually, but they have a chance to cast at the end of the turn, which is a per-copy chance so having 10 ethereal copies on a creature could mean 10 auto-casts of Fermented Hops, in theory). Affliction is good for when you run into weird things you can't kill the normal way. Spectral Rain can help affliction's effectiveness when you roll damage debuffs like burning.

ALTERNATIVELY FOR THE CARRY: Forsaken Bonescraper's trait on the artifact (like rank 4 in Damarel, iirc). It should do pretty nicely as a substitute. If you're going for RI5 and enemies are resurrecting a lot (and you're not dying to reflection damage), consider Frostbite Wight.