Well that was a long time before Volo, or even the forgotten realms for that matter.
Anyway why would any of us want to kill the Nilbog? It's funny and fantastic. Mind you I'm a bit worried someone else might hurt it. You know what the <targets> do to goblins.
(Bard deception vs. Nilbog...
iirc The original Nilbog started degenerating when you hit it (like a troll, but backwards). So you just had to keep it from harm (and especially fire and acid) until it died. You could use charm or hold person. But the usual approach was to offer to burn it with your torch if it promised to...