For tactics I'd say keep in two man squads and cover all angles, make sure your buddy's back is covered and he's covering yours. The hardest enemies would be Chieftains (with invincibility), Hunters, and Drones.
Edit: Although if your prepared, you help each other out, you communicate, and you stick together even the toughest enemies will look like playground bullies. =)
~Hunter
[Edited on 06.25.2009 1:30 PM PDT]