Yeah, unforuntately that is just how unstoppable works. We didn't want to mess with the badges original code too, so it ends up making a unique, if strange, challenge.
If you are running the up to date version, Lavavein should auto-detonate after 4 hits. If you are strong enough you can just kill him regularly before that, or if you are feeling creative you can make him explode. Just be very careful if you aren't a melee build...