I honestly thought that the quest was oriented towards going in there and beating the beast to death with mah fists. Only later, using the wiki, was I able to discover that there was another intended resolution.
I don't really care about how this quest is resolved, but if it is not intended that The Beast be killable, then it should be immune to resistance lowering debuffs, and maybe there should be something that indicates how to properly resolve the quest using the intended mechanisms.
Edit: I know NPCs indicate the Beast is indestructible, but when did that ever stop the PC from doing anything else other characters find impossible?
I don't recall any NPC saying "Hmm, well maybe if we could determine the biology of this thing, we could discover how to kill it. Hint hint, wink, wink, nudge, nudge."