In other words, you too acknowledge that the thief is simply a ton more powerful than the elementalist. Why are we arguing?
If the thief can match an elementalist's damage with his autoattack and mindlessly spamming 1 button while the elementalist is keeping mental track of a ton of things that don't even show up on the UI (like cooldowns behind other attunements) while his damage comes from avoidable/interruptable combos then yes, the thief is simply a better class. Everyone in the world has limited brainpower, even the best player in the world, and no one in the world has enough brain to keep track of everything in a PvP battle. If the player on the thief doesn't need to dedicate brainpower to pulling off damage, it means he can keep track of a lot more things and he will simply perform better.
If it is harder to pull off the same results on a class, then that class is weaker, end of story. It would all be fine if the elementalist was a hard to master class that becomes really powerful if you master it but it's not. It's just a class with a harsh learning curve that becomes less weak after you work on it. An elementalist player will always be inferior to a thief of equal skill.
Well I have 5 characters since in this game you don't need to level to PvP.
I am a FotM player but I have a special weakness, I get obsessed over experimenting with weak classes trying to make them work which is why I've spent time on silly stuff like trying to PvP with an elementalist.