I'm not sure what spell you mean, but if it involves a ranged weapon attack the Archery Fighting style is a plus 2.
If you go sorcerer you can use True Strike with Quicken spell to get advantage.
Knocking them proneor asleep gets you advantage in melee range
Paralyzing them gets you autocrits
Bless is a D4, bardic inspiration is a D6 (or more), both of which you can have other characters use on you.
Heat metal their shield to get them to drop it (not sure if this works in BG)
Arrow of Acid gives them -2 AC
There are probably more, but those are the ones off the top of my head.
