@Zandafolf I can't help you with the fire caster build, although I can say I haven't seen too many of them running arenas and BGs lately.
About the CC, there is actually a lot you can do. The obvious solutions is, as you said, join them. Get all the disgusting CC they use against you and out-CC them. The most popular choices are:
-Shadowfury (instant AoE stun, yes please.)
-Psychic Scream (same as above, fear)
-Death coil (instant ST fear with a lifesteal)
-Cyclone (with 2/2 duration talent) or Hypnosis (Hibernate) from the Epic RE or polymorph
-Hammer of Justice (ST stun)
-Curse of Exhaustion and Frost Nova/Entangling roots (from the buff) for rooting and kiting
The other solution is to get CC breaks. These include (but are not limited to):
-Tremor totem against fears (you can get your healer to take this)
-PvP trinket (almost everyone should have one, even humans with their racial only shares 30 sec CD with it)
-Berserker Rage for Sap, incapacitate and fear
-Blink for stun break (this is a must have)
-For silences you can take aura mastery or icy veins, but they must be used preventively
-Power Word: Death can be used to break non-instant sleeps and polymorphs if you see the enemy casting them at you, because the damage you take is delayed.
-Divine Shield breaks all CC and is a good thing to have, along with Divine Protection and the talent 3/3 that buffs their duration by 3 sec. Just watch out for mass dispel and shattering throw.
-Hand of Freedom breaks and makes you immune to roots and slows
That's all I can think of off the top of my head right now. Of course, you don't have to get EVERYTHING on this list, but having 2-3 options is good, and combine them with 2-3 CCs of your own, and you're good to go!