Let's Play X-COM Enforcer Mission 32
This is by far the hardest chunk of the military base, and possibly of the entire game if we discount The Worst Level Of The Game. The almost total lack of cover while dealing with boss-tier enemies that are swarming the area is pretty insane to keep alive with, and then the boss is just nasty. If... a bit unoriginal, being an Arachno-something that's a quadrupedal machine-thing that shoots at you in a shooter game...
If you told me this was meant to be the final level of the game, I'd buy it, but it's actually not. Getting pretty close, though.
This also ended up showing off that 75% Stealth is already enough to get pretty much every enemy to totally ignore you, even while you're actively shooting them. Still no idea how its mechanics actually work.