I got the beetle down to 30% health and then flashbanged it. First try, I exited combat immediately (press ENTER), but the beetle remained hostile. Unable to engage conversation. Second try, I skipped my turn. Nope, did not end combat. No way to complete the quest, unless if I go out and hunt for a different stunning or incapacitating method.
PS: that's one of the worst designed quest I've even seen, btw. The whole quest hinges on the player being able to end combat with the correct circumstances. If the beetle has a doppelganger, then it will not work. Beetle will run away, so it's a frustrating chase if you didn't get it right. Quests like these practically screams for save scumming. This can easily be fixed by having the pacifier be an active item, or automatically start a dialog once the player successfully incapacitates the beetle. Terrible design decision.