i cleared this first try with mosin, suomi, colphne, and groza. i just stayed in the cover in front, letting it heal because i have no idea since it was the first time fighting against it.
i just break shield asap by just focus fire, suomi, colphne, and mosin has element advantage(?) to it anyway
spiders didnt dmg me that much as long suomi's shield are up and the next turn usually is the time to refresh suomi's shield. i tried beating the first spider that spawned with mosin since boss shield is up, so mosin wont do any damage to it. the second one that spawnd, i let is explode and it barely do any damage to colphne. the next spiders i totally just ignored because they really dont do any dmg to the team.
boss can't outheal mosin's dmg as long as its shield broken and skill 3 + ultimate deals a huge chunk of the boss' hp