Gigapocalypse (UP)

You need to defeat Octopy 5 times to unlock this achievement, which is super simple, but really grindy due to RNG for its appearance. It is found in the Metro Dome 2 world and it will appear in the background looking like a giant octopus thing. Just shoot it until its eyes turn to X's and it sinks below the deck you're walking on. Once it is dead, can quit back hitting and select "upgrade" from the menu to go back and start another run. If it doesn't appear by the time you get outside the moon base, quit out and start another run.