Halo: Reach on Heroic. It's brutal sometimes. I'm at one point where there seems to be two SuperBrutes (whatever they're called, the kind that have shields). One has a gravity hammer, one has a plasma canon. The plasma canon guy will one shot you. While your back is turned. Without paying attention to anyone else. I've died at least 6 times to green explosions out of nowhere.
Their AI isn't bad. They'll prioritize you (you are the biggest threat, after all), they'll retreat if you drop their shields, and keep moving and find cover if you're sniping. That's about as complicated as it gets though. Note that friendlies are still dumb as rocks, though.
Apparently some of the unlockables are tied to Halo Waypoint. Which tracks and ranks you based on your Halo achievements. Maybe I should get around to completing the Halo Wars campaign. Odd phenomenon, not playing games you helped make.