It looks like you have already removed the xcap, otherwise it stops at level 11/12/14.
I have finnished the game with a fighter/mage/thief. You must backstab, it is the greatest ability of the thief. To make the game a little easier, you can also sell the wands that you got in the beginning and steal them back, then you will have 50 charges each.
I prefer to distribute my proficiency points this way:
katana++ (Celestial fury)
flail++ (Flail of ages)
short bow++ (Tuigan bow- great in the beginning)
later in the game:
long sword+
single weapon style++
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