From your wording, it sounds like you're after a way of casting at the enemy arena members.
Code:
#showtooltip
/cast [@arena1,mod:ctrl,nodead][] Wind Shear
This will target enemy arena player 1 if you press ctrl, else work like normal. I assume you meant the shaman interupt spell instead of the npc ability windshock.
If needed, you can go crazy and nest these:
Code:
#showtooltip
/cast [@arena1,mod:ctrl,nodead][@arena2,mod:alt,nodead][@arena3,mod:shift,nodead][] Wind Shear
ctrl => enemy arena member 1
alt => enemy arena member 2
shift => enemy arena member 3