1. It is always better to concentrate your firepower. You'll take down the baddies quicker that way. Target your opponent and then hit ctrl + spacebar to call out the shot. If the other members of your gruop hit T they will all attack your target. This is especially effective with NPC's.

2. Vs Charr. Take out Overseers, Martyrs, Mindsparks and Firecallers in that order. Overseers are Monk/Necros, Martyrs are Monks, Mindsparks are Mesmers and Firecallers are Ele's. You don't want them healing their warriors as you'll just waste your strength.