Thorim arena works as follows: There are four mob types that will jump into the arena
1: Dark Rune Champion. These are the tall vrykul that whirlwind. Use a tank to pull these out of the center, and have 2-3 range focus on these, and only these. This ensures whirlwind will not destroy all.
The rest of your arena group focus on the following:
Main Priority: Dark Rune Evokers. These should be your absolute top priority in killing in the arena. These spam a HOT on themselves and other mobs that heal for 6k per second. Take these out or you will be over run. You should have a melee with a /target dark rune evoker macro, and other melee assist that melee on the evoker. They dont hit too hard.
Dark Rune Warbringers. These hit semi hard, and should be picked up by your second arena tank. They cleave(I think?) and give a 20% haste aura. These are second priority to evokers
Dark Rune Commoners: 25k HP, spam low blow (making you hit for less, if they dont die fast enough, your tank will not gain threat any longer, due to having 20 stacks of low blow)
Mobs come in waves. Wave will be something like....
Dark Rune Warbringer jumps in
2-3 seconds pass, Dark Rune evoker jumps in, and a dark rune champion
4 seconds after that, about 10 commoners will jump in.
This series will continue until the tunnel group finishes the guantlet.
In short, your arena group will have one group dedicated to champions (1 tank, 2-3 range)
and an evoker/warbringer/commoner group
(1 tank for all the mobs, melee focus down the evoker, then aoe down the commoners, Warbringers arent too much of a hassle, and will generally die in all of the aoe/spam anyway)