I'd just unlocked one of the rangers stay-stealthy skills and managed to take them around the outside of the map, and the rest didnt push up very far at all, once i'd got a view of the tower, i could snipe it and do 6-8dmg (16HP). I ended up having to take my last shot with the ranger because drop-ships were getting too close, but because i hadnt pushed out much, i was able to do plenty of overwatch and take my move/shot more strategically, as i wasnt going out and being spotted.
On the sprint back, i had the huge berserker thing chasing me and was 1 tile away the whole time. I managed to use the little robot chap to protect on turn, causing 3 shots to miss, but with the cooldown i couldnt do it the 2nd time and one of the advent guard ppl got a hit in
Would have been a right mess if i hadnt unlocked that skill the previous mission (ive not done a mix & match build yet either - just following 1 branch)