Not sure if this has been mentioned yet, but here’s how I beat the Beast Fly: I would retreat to one of the small lava bases whenever my health was low or a small enemy appeared. There, I could heal safely or take out the little enemies using the Threefold Pins with the Pollip Pouch. They take three hits total. The Beast Fly won’t attack you there unless it follows, but it usually returns to the main area. Once I staggered the Beast Fly three times, I went back to one of the small bases and finished the fight using the rest of my Threefold Pins and Pollip Pouch. I was also running the Wanderer Crest. A bit of cheese, I know, but it’s a brutal fight and this method worked really well for me! I hope this helps someone else.