For what it’s worth, playing on ‘Experienced’ difficulty, I found the battle MUCH easier when taking the guns out first. I probably lost a dozen times attacking the tanks first – there was just too much for my squad to deal with once the e-holes opened if both guns were still in action.
I also think there’s an element of luck involved. On my first attempt going after the guns, only 1 e-hole w/ 2 Wretches opened. All previous attempts, 2 e-holes opened with a total of 6 Wretches. Additionally, I critical hit 3 times on the first turn so I was able to take one gun out early in turn 2. The second gun went down on turn 4 and I took no damage for the remainder of the encounter. It took many turns to get the tanks, but it was easy to avoid the stomp and rocket attacks and the occasional adds. One small tip: don’t be afraid to use the pistol when your main weapon is out of ammo. It does only minor damage but still forces the Brumak to spin, potentially exposing the tanks to soldiers on the opposite side.