Increase HP isn't the answer.
Increase the Defense, reduced damage by let's say 95% of all attack for the first 3min, then it decrease over time.
With that kind of change, it would make the NM last longer.

Also, the paper drop, if you claim the NM, it make ppl pull really soon.
If they give the paper only for thoses that have not the claim (good way to troll out the players), it will last longer too, maybe indefinitely longer until another one NM pop. ^^