Passerby has you covered. If you follow their advice you should do well.
A couple of things
1) change the way you think of Shadowheart. Think thief-cleric outside of combat, caster cleric inside combat. Melee is almost always the wrong option for her.
Guiding bolt is her go to spell and a cross bow is the default weapon. Only use sacred flame if the enemy is prone (if say Gale cast trip or the floor is ice)
2) light. Wyll and Gale won't be able to hit anything without a light source. Have Gale hold a torch.
Other things
-- getting burnt by an arrow that misses is infuriating, needs to be removed from the game.
-- Did Minthara cast the mirror image spell? If so that could explain the low to hit chances. The Larian interpretation of that spell is weird.
