Hyper Abyss Foscari Achievement in Vampire Survivors

Refer to Genevieve Gruyère (5G) for how to access this boss fight. Basically, you need to break the Seal of the Banished in Abyss Foscari. Doing so frees this boss, and you'll get sucked into a boss fight in a new area with all your weapons wiped out (but you'll still have your items). The Boss will be stealing life from you, and you're forced to rely on the drops of Prismatic Missiles. At least, that's the only weapon that was dropping on the ground around me and the fight. Within a minute or two, I had picked up enough Prismatic Missiles to have it fully leveled up. I also got a Magic Wand and maybe another weapon. I'm not sure if the weapon drops are random, but basically you want to be constantly looking around you for weapon drops to add to your arsenal so you can actually damage the boss. In addition to the boss, those red fire exploding enemies from the second stage of the game show up as adds during this fight.

What was very strange about this fight was that I had no health bar after I got sucked into the fight. I have no idea if this was a bug of intentional, and whether or not I was invincible or just couldn't see my health. From what I could tell, I was never close to dying, I just circled around the boss, picking up all the weapon drops I could find, and just waited until the boss finally died. When the boss dies, your run will automatically end and you'll unlock this achievement.