I use this on my mages. I take no credit for it because something similar is posted on this site. I use this on my 3 mages, all hotkeyyed to 1 button. It essentially rotates polymorph based on what my Priest has targeted.

MAGE1:
/target [target=focustarget]
/castsequence reset=5 Polymorph,,

MAGE2:
/target [target=focustarget]
/castsequence reset=5 ,Polymorph,

MAGE3:
/target [target=focustarget]
/castsequence reset=5 ,,Polymorph

I don't use the "/assist focus" to assist my focus because if the mob is in melee range, i don't want them being smacked out of polymorph.

Make sure to include the comma's! Also, I put the reset in there so in case of a resist/stun/whatever, it rotates to the next mage to cast it.