What am I doing wrong? (WHM)

[WhoIsTheGuest]

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.

[RhazeCain]

The question was about long boss fights. To date, I have not seen sleep used during a boss fight. Ever.

As a tank, I'm not sure if that's because things are immune to sleep or just that they have to die and there's no point in waiting. There may be times when sleep is valuable, but that would be strategy specific to certain boss - not general answer.