When a Nightmare boss defeats you, the result is more useful than “need more gear.” Ask what actually happened. Did you run out of recovery resources? Did you stand in a pattern you could have avoided? Or did the fight simply last too long? Each answer points to a different fix.
Change one variable
For the next attempt, change either your positioning, a defensive timing, or a damage window—not all three. That makes it easier to tell whether the adjustment helped. NC expanded Nightmare with more bosses and a new Layer IV in Chapter 1; later stages may reward prior-stage clears together, but the exact reward shown in your client matters more than an old route.
Know when to leave
After several attempts with the same failure, step away and do something that builds your character or your understanding. A solo mode is a laboratory, not a sentence. If the boss is consistently reaching a final phase, more practice may be valuable; if you cannot survive the opener, review your setup before pouring Kina into an unrelated upgrade.