Overshield comes from using melee, generally speaking in PVP you rezz, then backhand slap the guy who killed you to get your shield and then pewpew. Normally I get to kill the dude who got me, and then I'm taken out by someone from accross the map because you light up like a christmas tree when radiance is activated. I rely MUCH more on my super as a voidwalker in pvp, but maybe thats because I play a lot of control. As a sunsinger its just a revengebutton, so you kill people AFTER they get you, not before. Messes up your k/d ratio :P