Magma Armor
The Pokémon is covered with hot magma, which prevents the Pokémon from becoming frozen.
Flame Body
Contact with the Pokémon may burn the attacker.
Its lava body can cool and chip away at times, but a magma bath will heal it right up.
It's said that in ancient times, when volcanoes were more active, there were many more Slugma than there are now.