Working on my G4 challenges atm, done all the DMR, Quad Rocket and Autopistol stuff.
The Charge Rifle Critical Hits are an absolute pain though! The enemy Titan's shields have to be down, and you have to hit a tiny red area on a moving Titan, that's quite often surrounded by other titans, smoke, and explosions, all while trying to aim someting that takes 4 seconds to charge before it actually fires... so it's a case of aiming roughly in the general direction and praying they don't step 6" to the left between you aiming, charging, and the beam hitting them... >_<