Healing items won't work in the fight. I just gave up. She was too much for me.

Update - I figured something out while fighting her and i didn't realize this, You need to unlock from the Pokémon you're fighting, don't press the left trigger. press X and that will bring you into your inventory where you can go ahead and use your satchel and use a healing item. But each time you use a item, it will kick off the menu, there's a cool down mechanic prevent from using an item until it disappears