Giselle is a pretty solid unit considering how well she meshes with other water units. Bolstering EWD (elemental weakness damage buff), BB on spark assuming ES conditions are met, and a pretty solid single target nuker.
If you're worrying about Atk stat, note that Giselle's second attack on her BB and first attack on her SBB have HP-scaling, making it easy for Giselle to cap Atk.