This was last boss for my trinket, also I'd say most tricky one as well;
Have all melee on the boss all the time; druids, hunters and mages should create macro:
/target Nightmare
/cast Moonfire/Multishot/Fire Blast
At beggining of fight put all locks and one hunter on mana drain, when he's on low mana, ask one warlock and hunter to keep draining, rest DPS. Every now and then he'll do AoE sleep - this also clears aggro from MT for a moment so have offtank far away (so he doesn't get put to sleep) bringing boss back to MT. Also whenever he puts people to sleep, he usually summons 3 big adds, they move very slow and have little HP however they hit for 4-8k; that's where the macro I mentioned comes to use. Sleep doesn't last long so casters should have enough time to get rid of them.
Afaik horde can put totems all around melee, adds should go for totems first giving raid more time to nuke nightmares.