The easiest would probably be to target the mob with your main, then casting "/cast [target=focustarget,harm] Polymorph". If you need to sheep again, you would have to target with your main again and do it again.

If you wanted something more advanced, that could remember what target to sheep, then you're out of luck I think. There was a thread a while ago exploring some options of freeing up focus for CC.

Focus-free assist