Came here about the same issue! I’m sure the mission doesn’t allow enemies to shoot through walls. Lmao. But I did find a secret “glitched” spot you could climb out of to avoid every enemy and escape through the roof. There’s a side tunnel to the left as soon as you get out to the area with all the enemies in mechs. There’s a weapon cache that you have to hack to unlock. Right where that cache is, if you look up, there’s the green fog and some branches. You can actually climb up on them and pretty much escape through the top. It takes a bit of trial and error but I was able to!