Shadowheart's sacred flame sucks in every imaginable way. She can barely hit with it, and it doesn't do a whole lot of damage when she does manage to land it. Personally, I've ended up seeing her as a rogue that traded backstabs and cunning actions for some cleric spells. Thus I go dagger/shield for melee and light crossbow for ranged. And she does lockpicks and traps too, since she's got proficiency in sleight of hand and she's got guidance on top.
And in that style she's actually rather useful. She can tank with good AC, she can deal okay ranged damage with the crossbow, she can play a decent game of hide and sneak, and when needed she can lay down some very decent single target hurt with her spells. You also don't feel bad spending her action on tossing a potion.
