I have levelled up the streght of both my Archer and my 3 mages, and this is my experience:
The archer I levelled up early in the game, I went very fast and for each strenght point he gained, I gave him a better weapon.
The mages I levelled up considerably later in the game (after the Goblin dungeon), but it went relatively fast even if they were using staffs. Contrary to what Angus experienced, my mages hardly increased at all in dex and int using staffs, and the pace of strength increase did not go faster when I tried to use axes instead of staffs. However, they weren't strong enough to use axes or swords that dished out the same amount of damage as their staffs did.
According to this
FAQ your stats develop according to the following:
_____M_______R_______NM____CM
STR__64%____25%_____9%____13%
DEX__27%____62%____18%____17%
INT___9%_____13%____73%____70%
M=melee, R=ranged, NM=Nature Magic, CM=Combat magic
I don't know if this is correct, but according to this FAQ, this is how your damage-based XP distribute among your stats.
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