You're right, I think RJ mentioned that XP for mobs is based on highest level in group.
Casters in PVP can do okay if they get the first shot. If I can knock them down or stun them then I can usually lay in enough damage to make them run. Rarely do I kill one as by the time they recover they don't want to risk sticking around. However, if they can stun/freeze me then they can sit at a distance and just wreck me. Sometimes I can use the 'charge' skill I have to move quickly at them from a distance but it needs to be a straight line shot.