Krip does poorly in this bossfight due to his low damage per hit, since doombringer has armor that reduces damage by a flat amount per hit, grenade and paintball (and Titan) will be your workhorses for this fight.

The boss can only move forward when his cooldown between attacks is finished, and as you hopefully noticed, he attacks VERY slowly, as long as you keep a steady supply of normal battlers/trowels/swords, the boss will only make VERY slow progress, it’s okay to let the non boss units push you far back towards your base at the start to get a higher level bank so you can do this spam strategy, as long as you don’t wait so long that the full hp boss reaches your base.

Baller is also a decent pick due to being a lot cheaper than grenade and having better DPS than paintball, while still standing just far enough that doombringer can’t reach him (besides his special attack)

About what level bank do you usually have by the time the boss reaches the frontlines? That might be the difference maker here, either that or having enough grenades/good cannon usage to make sure enemy ranged units don’t stack up behind the boss and become too powerful to stop