Just did this on Classic with saves, and with the late-game mob types, including like half a dozen Commander-enhanced Mechtoids, numerous Mutons, Elite Mutons, Cyberdisks, Chryssalids, Heavy Floaters, Berserkers and two, count 'em, two, Sectopods! Not to mention waves of Sectoids. What a great mission!
Failed the first time, as I didn’t really notice the utility of the balcony until too late. The other reason I failed was because I didn’t realize that the game picks your first 5 or 6 top guys automatically - and equips them however they happen to be equipped at the time, which means if you’re fond of using the auto-unequip function and regularly rotate your troops for missions, be careful, and manually kit your best 6 out shortly before the mission hits. Facing mobs like that with accidental laser rifles and LMGs is no picnic, in fact impossible, as I found.
The first couple of waves you pretty much have to be in the killbox, but they’re handleable, and you get some breathing space to position yourself before the seemingly endless last big waves from the mec bay side.
I found the best spot is there’s a wee jutty-out bit on the balcony, one side of which faces the place where they mostly come in. Only half-cover, but with judicious use of amped-up Support smoke, most stuff misses, and if you’re facing the endgame guys, you can be pretty much kitted out with Plasma everything and Titan and Chitin anyway, so plenty headroom for mobs to chip away at you and not kill you. Just remember to split Overwatch and reloads in breathing spaces.
I don’t like deliberately sacrificing anybody if I can help it, but even used judiciously, the security guys die like flies. They died bravely!
Amusingly, the last couple of Mechtoids wouldn’t come in, they were cowering under some stairs and I had to find them. I think when their Commander was killed, they lost courage :)
I think an alternative strategy for that last bit might be to sneak round and kill the Commanders, who are mind controlling lots and lots of Sectoids and armoring the Mechtoids. That would kill a lot of irritating birds with a couple of stones, so you could concentrate on the big stuff. You can sort of see the brain-waves all converging on slightly bright spots, their powerful Psi brains :)
But it’s probably easiest just to set up the killzone and let them trickle in of their own accord.