15:

Well, since every 15 who sees Blind's thread has my build now I suggest using it as well.
Use a thud gun, premium gear, alway kite. Don't try to rush things, be patient and wait.

20:

Com? Why lol..
Anyways, for com use a brisk shield (pink) and always stay close. Don't waste your mass, check out quan's guide for awesome tips ! (I don't recommend com)
Op-
Well, mostly 20 ops are now damage ops.. So, waste their blurr and kick their booty. Be patient, don't always rush in. Careful when you're around the pillars, most ops bug you. If possible, avoid circling pillar fights.

Anyways, if I was you I'd go to 30 PvP. It's a lot more fun, and their aren't as many cocky people like l20 PvP lol.