The true answer: You didn't use Repose...

Repose is pretty spammable and every effective usage will save you multiple times the cost of Repose.

Macro it in this way:
(Start step 1 after the MT marked the Attack1 target)
1) Mark a different target with Ignore.
2) Shout in party that you are going to sleep the target in the next 2 sec.
3) Wait 2 sec.
4) Cast Repose.
5) Repeat & Rinse it on every monster except the target marked Attack1 by the MT.

You may even have extra mp left to do DPS :P
After the monster wake-up, if the tank didn't grab it, then just knockback the monster and Repose it again.