play it on professional. massive XP boost. especially if you're good at it and get lots of zombie kills and find the intel. Just playing Versus only nets you an average of 1000 XP per match. Better off just playing on Pro, you even get XP points just for playing at that level. --- PSN: Marshalljmpmk2 Current Number of PS3 Platinum Trophies: 25
Or replay the mission with two tyrants over and over again (about 10k xp per run through).
One thing I wanted to try was play a private match with at least two players (more if you can get them) but don't kill each other.
Have one player spam the pheromone ability to draw enemies to you then kill them all. Rinse and repeat for the full match time. You could play heroes as I know Tyrants spawn in that mode, killing a handful of them in a game could get you a lot of xp.
Could be fun with two teams of four holding down one area of the map each and never engaging the each other.