Tracer missle, is your best single target damage. On groups of 3 I will start off with Death From Above, which at that level usually kills anything not silver. I also like to do Death From Above followed by a Fusion Missle i think. There is also a gtae that I can not remember the name of, its like a scatter shot where you hit up to 3 mobs I think in the area, i use it if Fusion Missle is not up. Clean up anything left over with tracer missle if health >30%. If your facing two champions, electro dart 1, tracer the other mob. after electro dart wears off pop your shield. Heal if needed. When you get concussion missle? this will help out a lot, concussion 1, kill the other. When dealing with elites, if single, have mako start the fight, this gives you at least 2-3 tracer shots before you pull, pop your shield if needed, and mow it down. This should work well at 21