ArchLord

Personally I took Mu Mastery for AoE Immobilize and the resistance shield. My defenses are already high for smashing, lethal, and energy (I want to say about 45%, maybe higher) so the resistance bonus is nice for the attacks that do get by. Plus the AoE Immobilize helps to keep the mobs under control and prevent them from scattering from the assault bots missiles. Just my 2 cents.