He can just become a bacteria that has high resistance to heat then. Why not try to take it to the sun or black hole, that would be too far fetched though
Imagine instead of Piranhas, Kars turned the feathers into venomous snakes or spiders. Joseph would've died. But knowing Joseph he'd have another asspull for that as well.
This sequence is awesome, but I have to wonder how he managed to trick Kars with the dummy in the parachute, or how he managed to get into that plane so quickly, or how he even knows how to pilot a plane