With Nergal, you just got to be careful while in Pang Pang. Try to attack close only if he's going after another party member, but if he goes after you, just keep your distance. Spit an occasional attack as you retreat because if he's really after you, he'll run into it and get hit. And keep a sharp eye out for the sparkles that outline his 1 hit KO attack. If he hits you with his hax range attack, you should hurry and get out of they way if you are far away enoughfrom him, but staying down until his next attack passes over seems to work if you feel his AoE is coming and you can't outrun his range. Sometimes I turn to Cool Pang pang and shoot stuff at him from an entire screen away from him, but that's not really efficient xD.
Also, try Light shield potions. They are not as useful as Flawless, but are easy (IMO) to get (get 8 for 5 of any fishing bead) and can really save you in a pinch. I think they also have a separate cooldown from Flawless, but not entirely sure. Just don't attack until you're safe or it'll cancel .__.;
Aurum211: Myth Healer 7th RB (350% exp rate RAWR)
Gummygumdrop: Myth Lime 4th RB (on leave for now...)
Mabinogi IGN: Zinc210: total lvl 7xx