Prime mode is unlocked after you level up all the four classes (Infiltrator, Destroyer, Titan, and Scientist) to level 25 (the max level) in multiplayer. Unlocking Prime mode resets all your class levels back to 0. After you reach level 25 in all 4 classes in Prime mode, you unlock the Prime 100 head pieces for multiplayer character creation.
I must confess, I've only played the first mission in single player mode. Most of the time I play multiplayer, cos I find it tons more fun than single player. Damn, multiplayer Team Deathmatch games are so fun. I've never experienced so much joy and excitement blowing things up as a giant robot. Woohoo!
|Perceptor, from the Havoc Multiplayer DLC|
|Optimus Prime G1 skin with Striker head|
|Megatron with Prime 100 head, shot 1|
|Megatron with Prime 100 head, shot 2|