The Brumak is encountered toward the end of Act 1 Chapter 6: Into the Fire.
As the fight starts your squad of 4 is split by the sides. I recommend to take out the machine guns off the Brumak as it limits its attack pattern to simply stomp and missile barrage.
Try your best to essentially keep the Brumak in the middle and chip the health down. You get a checkpoint about halfway towards his health bar.
As you destroy the tanks on his back there will be Emergence Holes. That pop up with wretches. Take them out quickly so they dont cause any issues.
I did notice in my play. I popped the Piñata achievement As well. So if you havent popped this yet. This could be a solid and easy opportunity. As that achievement requires to kill 1 single unit with iy taking damage from all your units in one mission.
(Sorry im new to formatting so im unsure how to add the Piñata link and to writing guides. Apologies if i missed anything. And please do encourage tips. Thanks for reading)