Yes, the Minor Avatar of Sekolah can easily wreak havoc in your party if you don't plan ahead.
His melee attacks are devastating - he deals a lot of damage, causes "bleeding" to add insult to injury, and as if that was not enough - he has that annoying "swallow you whole" attack. It
does allow a save, though - save vs. death at -4, if i remember correctly. This is a good time to drink those *stackable* potions of Invulnerability.
As for killing him, it took me a reload or two to find that he is very susceptible to backstabs. The only problem is that he can see through invisibility, so occupy him with summons to prevent him from targeting your backstabber and thus ruining the attack.
Good luck.