Whatever level you are on, Damian will jump 9 levels (I was 29, and he jumped to 38).
Just before going through the final rift (or to be more accurate, after going through unprepared, dying and reloading) I had the DK unlearn all of his skills, and invest in summoning doll upgrades (levels and skill point transfer). After that I had the skeleton doll leveled up to 31, and learned Crossbow Expertise: damage, point blank and reload time (as an archer I always tried to max reload time, but a point or two in damage / accuracy / point blank was good enough).
With the other 2 dolls (or 3 if you managed to keep the Raanaar doll from act 3) summoned as a distraction, my hero (just recently switched to a bow) and the skeleton doll (with my previous crossbow) using power arrows, I took him out by avoiding too much melee combat (have whoever he attacks run away, while the others, set to aggressive, continue to attack).
Only having the one holy water for most of the game (act 2 quest reward), I decided to enter the BF a few times looking for more, and (after finding a nice bow) managed to get another 5 (all the BF merchants get one at the same time), to make permanent agility potions for my hero to use.
