Experience is based on average party level vs. level of the opponents. There is a sliding scale for this, so that if the group is up against higher-level monsters, they get more experience, and if the monsters are weak, they get less.
It is entirely possible for the party to get nothing or almost nothing from some fights in the game, such as in Kuldahar later on.
If you're multiclassing, I'd suggest staying with your party of four, which looks good enough to go through the whole game.
Nightowl2
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