If memory serves you could charm a mob that is a caster cleric wizard ect, and it begins casting certain spells before said mob is mezzed, and said mob is mezzed during it's cast then it sometimes could break a mez. This is not easy to duplicate.
So you just have to resick your pet on a mob it was beating on, if that mob gets caught up in an AE mez. There should be a pet command to ignore mez.