I'm not sure why the spell isn't doing as much damage as anticipated, or any way to increase the damage (other than raising the level or using a more effective spell). It is a relatively low level spell, so maybe that makes it less effective against high level opponents?
Pretty much the only mage spells I used were for healing, though, so I'm not sure what kind of damage should be expected.
Larian has not revealed any of the formulas used to calculate damage, and working backwards from the observed damage would be rather tedious.
I didn't play much of the after-game BF. In the main game just before entering the final rift I had my DK unlearn all skills and then invest everything into summoning doll upgrades (levels and skill point transfer). I then upgraded the act 1 skeleton doll, the only doll that can use a ranged weapon (crossbow).