Apocrea Gear

tbh, I really hate apocrea.
THE MAP IS TOO BIG
But that's not the problem... I just don't get what weapons/gears to use.
Okay, so... I guess I'll just put up weapons/gears that I find useful for this quest, and people can add more below :D
Gear:
BK Set/(Merc set?)
MSI allows fast runs and better chance of running away from harvester
Weapons:
Argent Peacemaker
-High bullet speed and bullets per round allows fast grave breaking
-Can run and gun
Final Flourish/BTB
-Quick swing for breaking graves
Warhammer
-Makes killing Lost Souls a lot faster and easier (Plus second dash allows you to kill other mobs [scarabs] even if they come out)
-Easier to fight trojans even if they come out
Voltedge
-Breaking graves with charge attack can shock enemies that come out (keeping mobs in place that come out)
Blitz Needle
-High damage to kill off Lost Souls or basically any mob
-Weakness: Cannot gun and gun
Shivermist Buster
-Freezes Lost Souls
-Weakness: Scarabs and Bombies that come out in Grasping Plateau are immune to freeze
Ash Of Agni
-Can wear down the enemy (especially scarabs which are hard to kill off)
-Can place behind you to kill off mobs that chase you / makes seeing small mobs easier
-Weakness: Strong when soloing mission, but perhaps weak when in full party?
Sleep bomb every grave, Hammer everything that comes out, trail of sleep bomb eventually sleeps Harvester repeatedly if you line it up right.
The harvester can be slept, as well as the souls.
You'll have to wait until the scarab attacks fully end before you can start getting near them, but with them sleeping they won't attack again and with most weapons you'll be able to clear them easily, and they don't always wake up after one hit.
If you're playing in a group with some players not knowing so much about how to deal with the harvester, one player could suicide rush the harvester every time it gets close, trigger it's attack but place a sleep bomb right next to them so it won't attack if the player gets captured, and another could just free them, then everyone can run off.
Not being able to rush around attempting to grab souls faster because the Harvester is going after you, sleep bomb usually helps and I'd imagine trailing it as a bait only would be pretty good too if you're not completely 100% confident you'll be able to escape it's attacks if it ever manages to get close to you.
Maskeraith to spook harvester so you can hide every time you get captured with no way out, and a heavy shield in case scarabs are attacking you while you're trying to stealth. Having the harvester near you while you go stealth will show where another player is, if there's a reason you'd want to meet up since it'll go in the direction of them.
Sleep bomb is always fun, regardless.

Hail Driver can somewhat be useful here in a sword/gun hybrid. I like using Hammer and Hail Driver and whatever souls that are drifting away from me, I single switch hail at them and hope the freeze will proc so that I can finish them off with Hammer.

Have you searched Google (or another search service) for something like "apocrea guide site:forums.spiralknights.com" (without the quotation marks)? That can help a lot.
The Apocrea weapons are not particularly worth getting. So the main purpose of playing Shroud of the Apocrea is the fun of the puzzles. To learn more about them, you might search for something like "apocrea puzzle site:forums.spiralknights.com".

It's my third year playing apocrea, and there isn't a single puzzle which I do not know how to solve .-.
Ofc, I think the same the same of obsidian weapons not being that worth getting except for the obsidian crusher, but I'm just running for sigils to try and get a grasping aura :D
But playing this much, I still do not get what weapons I should bring for the mission...

I could solo Shroud of Apocrea with just a maxed Electron Vortex, that's really all you need LOL.

I didn't realize that you were so experienced in playing Shroud of Apocrea. I don't have good advice on freeing lost souls. I always used Final Flourish / Barbarous Thorn Blade, because the first stroke was wide and decently fast. Best wishes.
I go with BK set, Rocket Hammer, and Polaris(If solo) or Arcana. The guns are good for staying mobile and warding off scarabs. I've tried using blitz but it feels very tedious to charge every time I need to break a tombstone, I have the same problem with bombs and voltedge. Flourishes work well on finishing spirits but I'd only use if I have an extra slot. Hammer works well if you wanna kill the harvester just for fun.
For sprite I use a Mask with swift steps. The Mask cloak can really save you from a harvester staredown.