Another trick is to make it look like you're retreating, while in fact you're flanking them. Most groups won't know what hit them when you start attacking their rear or flank. Also, use other players as a barrier between you and the attacking group. Just keep your team mates alive by healing them while you pick off enemy players one by one. Of course pure dps will usually get the job done too, but that will leave you as the only defence standing when the next wave of enemy players attacks (not to mention that other players will like you a lot when you keep them alive).

Normally I team up with a rogue. His job is to cc incoming players (especially aoe and fear spell casters) so they can't get near me and can't run when I open fire with my lightning.