6 vs. 6 in the hospital level would be nuts. That is a tight and brutal level.
I've been playing the single player more and it is hard in places, at least on guarded. The style of combat feels unique. It's not run and gun, it's not stop and pop, and it's not turtling. I think I would call it multi-tasking. Since you have rushing enemies, flying enemies and sniping enemies, plus spawn points and land mines, you have to face a lot of different attack styles all at the same time. I'm warming up to the alien weapons because they seem to do more damage.