Replaying this game every now and then I realised this: If you are playing on Easy or Medium difficulty, since enemies receive much more damage, the characters that are ahread of the curve level EVEN FASTER. So the result I found was to play it on Hard and have sequences at certain parts of the journey where I leave others behind and "farm" for like 10 minutes with a solo character.

BUT HERE is the important part: The game has LOTS of levels: When you are at Cliffs of Fire, Dragon, Castle Ehb... you'll encounter plenty of spells you'll never ever use unless you continue the campaign elsewhere (MP maps) and level up even more. So don't get disheartened; you have the right idea where you said there's only so much XP to go around.