Magi dress up differently than warriors. Magi tend to gear for hp, while warriors gear for damroll and hitroll(or at least my warriors do ;D).

Many invoker spells lag you only one round and deal pretty good damage. Dress up for hp(you should have no problems getting desert cloaks from Araile and glauruk-hide gear from poacher in that swamp southwest from Feanwyyn. Both are goodies, so you can request them if you are a goodie, but you should be able to kill them as well). Use the target character's vulnerability against him(ice against fire giant, fire against felar, etc.).

Invokers deal a lot of damage, but as magi they are also frail. Getting invoker shields at level 38 helps a lot if you bother to play that far. If you can't handle the invoker PK now, rank up to shields and you have nice elemental immunities(that finally allow you to solo-rank if you know what you are doing) and some damage reduction as well.

My advice to you is rank up to 38(with a group, of course) and start PK:ing after you have shields. I suspect that you'll just keep losing before that.