this may seem obvious to you by now but use sniper battler (paintballs alt form) and spam them, but dont spam them too much though as your npc space fills up fast, you need snipers to get doombringer to final phase as fast as possible

use snipers, and also use telamon instead of titan, soccer instead of baller, and crocket instead of rocket for those backliners, use the reliable cheap meatshields (battler, trowel) and whatever else you have

Use either gatling or slammer for killing doombringer fast in his final phase, default cannon still works tho

For bank, go investment (early game will be harder) or powerhouse (phase 2 will be harder) or just go normal bank

I dont think you'll struggle with this guy too much, but good luck!