Make sure the new head sees you first, then it’ll think you’re its mommy. Repeat until you have an army of trolls to take down the first head (and its new body).
Me the experienced player as a sorcerer that doesn't actually know a troll's weaknesses: Hmm, I'm not going to cast Burning Hands because there's only one monster, so I'll cast Hold Person instead! You can thank me later, party :D :D