Immortal monitor lizard?

[Uzkin]

One of the Boggard fights in Tranquil River Bend features a 1 hp monitor lizard that is immortal. What's the deal with that? Is it a bug or some odd game mechanic?

[Kurosu]

I dont recall such a thing, it is most likely a bug. The only immortal enemies I remember where some owlbears in specific locations.

[Uzkin]

It seemed to be the Boggard Warden's pet. It died once the Warden was dead, so perhaps there was some mechanic involved.

[Wither]

There is 'coup de grace' - a red button skill which you can use to kill incapacitate enemy. Works with troll and such. It will be splat of blood, sometime make it not loot able, though.
That or just use fire and acid. Torch or candid spell works.