Combat sim bugg?

Had the same thing, plus Wrex wouldn't shoot at anything, so I had to take another squadmate. Made the challenge really hard. I finally did it on the "Fatal Error" map where there is good amount of full cover and somehow Javik spawned on top of one of those blocks, where he had a perfect 360' angle to shoot, so I finally managed to do it. But you're right, the stun-engineers and vanguards are a real pain.