http://www.gamefaqs.com/ps2/459841-final-fantasy-xii/faqs/45730Use Holy weapons and stay in the corners so you don't accidentally zone out. Once you have the pattern down, just leave the game on. The only way I can think of you killing Negalmuur is if you get Confuse status. I've done this with lvl 10 toons; You have to go pretty out of your way to mess up the chain or kill Negalmuur.
Since you will die to abysteels (and they probably aren't available), I quote:
Head into the Nabreus Deadlands and find the Gate Crystal (careful, there's a
Crystalbug you have to get rid of first). From the crystal, go south into The
Slumbermead and hug the northern edge of the area. You will see a path that
winds around the back of the second rock you come across that is unmarked on
the map. Follow this trail and you will end up in The Fog Mutters. Keep
moving west and you will enter a small area called Overlooking Eternity.
This is the third and ultimate mass field of skeletons. You can generally
wipe out about 40-45 of them per visit. Return to The Fog Mutters and then
come back, and we have ourselves a party. It has been documented by both
myself and others that with Embroidered Tippet equipped, you can gather about
1200000 Exp (that's 1.2 million) in an hour. The numbers speak for themselves.