I've had good experiences not using direct assist for my target. So basically this is how I do:

1) All spells are targetted using [target=party1target] - for example /cast [target=party1target] Fireball
2) Target (on mages) is used only for poly, so I basically just assign poly using current target and I'll assign the target manually (I have F1 setup to select mage 1 target, F2 setup to select mage 2 target)
3) Then you can just use /cast polymorph to CC