Me and my brother beat a difficulty 12 cyborg retaliation mission.
For reference, that is the hardest possible combination of difficulty + enemy + mission type. A ridiculous number of enemies including tanks and walkers fall basically on your heads, in waves. You're supposed to bring four people, we did it with two. The game does not scale the difficulty at all with party size. We tried for like 3 hours before finally finding a setup that works, which even tho we both have DLC content the winning build was purely stock. We hid away in a tiny back area where we knew from experience no enemies would spawn. The two huge main entrances we covered in AT mines and slowing static fields, while I tossed the distress beacon in the tiny back entrance. No one would answer the call since the game was private, but its an indestructible building and it gummed up the works so no large enemies could get behind us. Finally, to deal with the horrifying amount of enemy armor, we both brought the reloadable AT launchers and did the assisted reloads on each other (we were very in-sync about this). My brother also swapped out the standard AR for a shotgun, to deal with the butchers which constantly fucked us up.
Without that setup, we'd make it to about 200-300 kills consistently. With it, we managed to win with a couple deaths and both of us successfully evacing. There were so many tanks that even with me spamming an ammo drop every minute or so, we would still almost run out of rockets between drops. WTF was that mission.