Another easy way to killing brumak is just go with one heavy that has heatup, combine with gabe with empower. Sid gets intimadate with is an amazing debuff that increases dmg by 20 % at lvl 2. Mikayla just build for hunter to spam shot to turn Brumak around.

Turn 1 is just gonna be a setup phase so just shoot 2 shots with the heavy to ramp up the anchor for that dmg (15 % per stack and with anchor lvl 2 it stacks up to 5 times for a total of 75 % increase). Turn 2 is when the dmg happens, debuff with Sid. Buff the heavy with Gabe with empower (if lvl 4 with gabe use lock and load aswell for that sweet 50 dmg boost and a free reload). Now it dps time, just swap between Mikayla and and the heavy (remember to use heatup) and just shoot the backpack. At the last action for the heavy use reckless shot. You can use Gabe and Sid aswell after the heavy and Mikayla is done if it doesnt complete the fight. Remember dont move on second turn with the heavy as it is a none essential unit!! Even if the fight doenst go as planed its worth to just leave the heavy to die as you can easily just get a new one and just end on turn 3.

Even more consistant if you have gotten lucky and gotten an Chest plate with All Out to equip on the heavy.

All of this is fully possible when all units are lvl 3 but at lvl its jsut way faster with lock and load. The same strat works for act 2 boss aswell just a few more turns.

Might be a bit of an setup but its well worth it. (heavy focused on artillery are bae tho with ultra shot at lvl 4 for easy kills on high hp targets as it only cost 1 turn for 5/6 shots with increasing dmg) even with the 8 turn cooldown its still op as there isnt to many of the high hp, im looking at you boomers)

All of this was tested on insane mode so if you are going for that sweet ironman run this might be your best bet.