Maybe you could stun lock him with multiple rdm/drk+chainspell like a suzaku/DL fight to prevent tp moves like nightmare from going off.
I don't see how multiple smn can kill it, nightmare will waste so many attempts to bloodpact and bleed the smn dry of their mp pool as they stand there unable to release their avatar.
A party setupsimilar to doing CoP 3-5 (but no cap now) would probably be the best place to start, blm (stun), rdm/drk (stun), drk/nin (stun) and any tank that can do something like flat blade (nin/war with sword mainhand) will give you enough stuns to stop nightmare >_>a
Just my thoughts ;x
XI Wiki

