My earliest lowbie PK successes came from invokers, wall of fire is just awesome, land 2-3 of those and watch anyone in their teens drop like a rock. Even in mid ranks the damage output is quite gnarly, first thing I did when I got done with Eldrinoth the PKless wonder was log in a level 28 invoker I had lying around and destroy a group of 3 with fireball spam. If I can do it, so can you.

Flying potions are easy to get and cheap, and you even have an anti-bash mechanism beyond enlarge/reduce with grease, which mitigates lag. Damage output with wall of fire exceeds what most teen warriors will be doing to you in melee, and if it's looking sketchy and they're having trouble bashing you, you can just flee and quaff.

I lost interest in my last invoker pretty much as soon as I got nova, so I can't really comment on post-shield invokers, but up until that point they're very deadly and really quite a quick and easy way to get a competitive lowbie PKer, perhaps even more so than warrior.