Perdita is one of the hardest targets for collodi to tackle. Kaeris is another bad one. That said it's not impossible. If he's brought papa loco that should be your target for the stitched, as his WP sucks. You say something about hitting for 3 damage on 2 targets, why are you attacking 2 different targets? Hit one enemy until they're dead as this game has no system for reducing effectiveness due to wounds.
You can also try using an arcane effigy which has some decent ranged firepower of it's own.
Against perdita I'd be more inclined to spend the first couple turns filling more dolls with stones (be that the effigy, stitched, collodi himself, or just the marionettes). Also make sure to have wicked dolls, they're awesome for the cost.