I lead a group into heroic Magister's Terrace everyday and after wiping about 15 times on our first, this is the final tactic e use on Vexallus:

Mage uses Ice lance on adds and kills them whenever they appear (seems to be every 15%) we dps the boss until it's at 70%, then dps stops until the debuff goes, whilst the tank (me) keeps on tanking. Once debuff goes  dps starts again to 40%. Once debuff goes, we dps to 10%, then burst dps to take him from 10 to 0 hp.

Been using this tactic the passed 6 runs and it's working fine =)