The one right after hologram boss? Or there's another boss after that?
You firstly shoot the girls near her to lower her shield. Stand at entrance, and fire charged shot at where you think they are, from long distance (your range is way higher than your vision field, so if you shoot something offscreen, you still hit it). If you have to wait for recharge (like me), or to use potions, run to the hologram room (it's not locked). Healthbar changes colour as you keep killing girls, until it turns red when only the queen remains. You then kill the queen, too. Be aware that she would summon one of generic enemies to her aid, which may be even more dangerous due to disrupting you charging up.