This way, the Demon Guards won't respawn, and at the same time, the Heart of Baal will drop. Baal has two guards, Niub and Niux: one is immune to magic, and the other can heal. Be sure to use ice mage to wear down the one that heals, then have the barbarian fight the one immune to magic. Finally, use invulnerability to pick up the Heart of Baal, which will nullify his invulnerability. Make sure to keep Baal in position so he doesn't get close to the mage or priest. Also, remember to carry the Heart of Baal while fighting instead of clicking to use it, otherwise Baal will infinitely respawn...
This game requires teamwork, and the healing pressure is high in the later stages. The ice mage lures monsters with positioning and awareness. By the way, there's a hidden plot. In the last level of the snowfield, there are two holes: one leads to Baal's stage, and the other is the hidden plot. Compared to the old demon in the hidden plot, Baal is just a child...