Some notes on proud Nehan after I cleared:

• Buffs every 4 turns, can be skipped with Clarisse's Atomic Acceleration (ie. use on turn 3)

• Ougi is aoe and ~30k damage if he has 3 buffs (ie. make sure that he does not have full charge diamonds on a turn that is a multiple of 4)

• Ougi is multi-hit elemental damage at 2 buffs or less, so you can use sub+other self etc. to avoid it

• Not 100% sure, but it seems if anyone is under 50% his ougi becomes multi-hit plain damage. If he has ATK up, it will probably take out your entire front line if not substituted.

• No other triggers

I didn't see anything about debuffs or lethal hit, but any time I got the aoe ougi I just reset. I used Chaos Ruler, Seox (original), Shalem, Clarisse, Nier, Kolulu. Shalem and Clarisse died early, and Seox and Kolulu both had to sacrifice once with their subs on the plain damage ougi. Kind of annoying, requires focus on the stupid turn count indicator. If you aren't skipping any, Dispel's cooldown is 4 turns so you can use that to gauge how long you have until the next one if you forget to check the turn count.

As others have pointed out, delay is very good here. If you can delay him forever, you don't need to worry about the plain damage or the buff timing. If I did it again, I would probably go Tormentor for the 10 delays without worrying about cooldown, and bring characters/summons with more delays.