You could use things like:

On A:
/cast [target=arena1] Hex

On B:
/cast [target=arena2] Hex

On C:
/cast [target=arena3] Hex

That kind of an approach.
It might force a trinket use.

I'd also think about a Wind Shear (round-robin)... exactly like the DK Mind Freeze (round-robin), probably at a Focus target...
Probably the (or one of the) healers.
If you're going at an annoying caster like a Frost Mage, you can change focus easily enough.