Not a guide, just a tip for those who are struggling with the Ophas that show up in larger numbers towards the final waves.
Using Ground Slam near them will push them back a little bit. Do this until they fall down the rotating blades, instantly killing them. It's advisable that you finish any nearby enemies before to make this much easier. You can ignore the Putti (the little flying enemies it spawns) for the most part as they primarily heal the Opha, but they could also attack you. This should save you a lot of ammo, time, and increase your chances of completing this, especially if you're using a lower leveled character.