When multiple monsters make it to the raid, the enchanters (sometimes with the assistance of bards or necromancers) will mesmerize (mez) as many of them as possible. Mezzed monsters do not perform any activity; they just wait until the mez either wears off on its own, or is broken because someone damages the monster. The raid can then concentrate on one (or a few) monsters at a time. When the other monsters are killed, the main tank will wake another one, the raid kills it, and so on. Ideally, the only person breaking mezzes should be the Main Tank.
This sounds easy, but in practice it is difficult and it requires every single person on the raid to pay attention all the time, especially to assist calls. Because a mez is broken every time someone damages the monster, it is vitally important to assist properly and only to attack the Main Assist's target and only to attack their target after the assist call is made. Every time someone breaks a mez, it wastes the mezzers' mana. Because tanks usually do not have time to gain aggro on mezzed monsters until shortly before the main tank wakes it, breaking a mez prematurely will usually cause problems with aggro as well; most often the monster will head straight to the person who mezzed it, because mez spells generate a lot of aggro.
Broken mezzes can easily wipe the raid or make the raid lose the event they are doing. When monsters are mezzed, they are most likely mezzed because there are too many of them in the camp, so - once again - it is vitally important not to break them.
A mezzed monster will wake up every time that the monster loses some hitpoints in any of the ways possible: melee damage, single-target spell, AE spell, or anything else. Abilities or spells that do not cause HP loss will not cause the mez to be broken, so it is safe (and often beneficial) to use taunts, debuffs, and things like that on mezzed monsters.
Sometimes the mezzers will need cooperation from offtanks, especially if there are many mezzable monsters incoming of if the mezzable monsters are likely to resist mezzes. In such a case, offtanks should aggro the mezzable monsters and occupy them until the mezzers have them under control. Because any damage will wake a mezzed monster up, it is necessary to use non-damaging methods of generating aggro - i.e. taunting, stunning, snaring, and so on.
Sometimes we are not mezzing monsters on a raid and we will just let the tanks deal with them. I must admit that sometimes the reason for not mezzing the monsters (even though we would do it otherwise) is that mezzes have been broken too many times by people other than the main tank, and under such circumstances it would be impossible or very difficult for the mezzers to keep recasting the mezzes, not to mention the fact that they would probably lose their sanity after a tenth or twentieth mez is broken on a raid. So, yet again, please make absolutely sure that you do not break a mezz unless you should do so.