After your encounter with the demons, travel to the mine and follow the demons.
After solving a few puzzles and getting deeper into the mine, you will encounter lots of mobs of which a few are creepers.
The creepers blow up in a similar fashion to a bomb so make sure you are in a straight line next to them. Get low HP by letting the bombs or other mobs hit you before going to the creeper.
If you fail to die by a creeper, kill the party, when you respawn the mobs are back so you can try again.