Oh man, whoever recommended Sniper Attack earlier in this thread, Thank You.
Very fun and good cR. Gruntpocalypse was enjoyable, but I could really only play more than 2 rounds if I had something else going (podcasts, TV, Netflix). Sniper attack uses all of the enemy classes, with infinite ammo. The power weapons keep it casual, but the AI keep you on your toes. Especially Skirmishers. Those guys can really wreck me if I'm not paying attention!
Lars and Team really nailed Firefight. The maps are good, but it's the AI that really does well. AI in campaign mostly plays defense (historically, less so in Reach), but the Firefight AI is aggressive as hell. They will get in your face and laugh as you die. <3