The third boss in Magister's Terrace is a nightmare on heroic without the right CC, or the perfect group of mobs to fight. First of all for the perfect group makeup you are going to want this:
1. Paladin Tank
2. Paladin Healer
3. Warlock
4. Mage
5. Any other dps/cc
^--That is the optimum group. But you are going to want a warlock, mage, and pally. This makes the third boss easy. Alright, now to get the perfect group to fight:
Summon everyone to the stone. Now disband, and tell everyone to walk into their own seperate heroic. Walk up to the first pull but don't do anything. Tell your party members to start typing these in:
/target Garaxxas
/target Zelfan
Those are the two demons. With a warlock you can banish the hunter, and if you have a pally you can pally fear the Engineer (considering you cleared the whole room (You can keep the Engineer permanently feared if you have 2 pallys, but its not always necessary)). Also the warlock can fear a third add.
You could also have a 2nd warlock in the group. This would allow for 2 fears, and 2 banishes. EZ mode.
This may seem like common sense but sometimes it is so ridiculously hard that if you have 2 demons cc'ed, and a sheep, it should be ridiculously easy. Sorry if this was posted before, just searched and didn't find anything.