At current you can abuse the AI to fix her with mirror image-Larians implementation is questionable at best, adding +9 AC instead of a chance of hitting images, and the AI doesen't handle it right. They simply refuse to target enemies whom they can't hit, even if they could destroy images, so they never attack shadowheart if you mirror image her.
Otherwise, you can give her heavily armored at level 4 to fix the AC issue. Bumps her AC by 2 (if you give her half-plate or +1 scale) to 4 (if you keep her in a chain-shirt) versus the best heavy armor in the game right now, chainmail.
Really, her STR and DEX should swap. If Larian wants to keep to the cover art, add a finesse spear for her. That's within the range of what a magic item can do, or heck, they could make spears finesse in general-no reason that breaks the game, really.
