General run guide:
Main characters are 4 soulslingers and 1 sythedancer
Soulslingers with dark shot and interception shot
Sythedancer with the collection and counter as well
Preferable 1 soulslinger with support skills to buff sythedancer
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Doctor "actionless" use your sythedancer next to 2 other gunslingers and use gunslinger buff to kill the boss before the boss can act
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Priest: summon order, gunslinger attack, gunslinger with quick action (move before the boss) sythedancer and support gunslinger
Keep buffing and using your sythedancer skill, walk towards the boss with both of your other gunslingers, use dark shot and damage the boss until he is dead, should kill him before his second action.
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Maiden: order is all gunslingers and sythedancer in last, use The collection with sythedancer, then dark shot on fast acting gunslinger and counter shot with another one, damage the boss and you should kill it with sythedancer's counter attack or dark shots. no more than 2 boss rotations.
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Child: luck galore, get closer and never explode, keep using sythedancer skill. counter shots and dark shots and hope the teleports are in your favor.
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Egg: use sythedancer to kill all the adds and just run with your 4 gunslingers towards the egg spawning point, when your units act after the egg spawns, start preparing dark shots and just nuke it before it can act once or twice (really depends on how much damage and critical strikes you can hit)
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Second phase
Move your gunslingers to the space in between your initial position and boss spawn point, after that, watch out for boss attacks with counters and nuke it before it attacks a lot, the more it attacks, the more time you lose. if he uses the skill to mark a unit, just explode and nuke it with everything you got.