I wish the game at least told me that the fight is extremely tough, like it did with the Solmen Keep battle or some of the Slum fights.

The Temple of Bane fight is not particularly difficult. Just make sure to neutralize Mace before he can cast Hold Person. Charm Person is an excellent spells for this occasion.
Plenty of Sleep spells as well helps, with primary targets being Orc Archers.
Also, I assume you have the NPC Cleric and a Hero hireling to help you?

Why not just load your last save? (You did remember to save, right?)

here's the thing: my last save was in the temple of bane.

So I was stuck there forever.

It occurred to me that, at least for the Gold Box games, there is a way out:
1. Export your party.
2. Start a new game.
3. Load the characters you just exported.

This will take you back to the start of the game, but with the characters you were using and had already leveled up (I believe you even keep their equipment). Basically, you can treat it as a New Game + feature that doesn't require beating the game first.

This also works with some other games, like Icewind Dale and Dragon Wars.