Genetic Disaster (UP)

The first boss in Genetic Disaster greets you as the boss of the third floor. He has two sequences in his boss battle, where the second one starts once he's below half health.

He is actually very easy to defeat if you're playing on easy mode. If you manage to get some high-level long-distance weapon, then that will be optimal. However, close-range weapons work too if you're careful to dodge his abilities, which we'll get into right now.

For the first part of his fight, he's covered in impenetrable armor which you can't shoot through. He has two possible attacks: Throwing bombs or shooting a single burst of pellets from his center, protruding from him in a circle. The first thing you'll want to do is pick up one of the bombs he throws (with the A button), and then throw it at him to break his shield. If you hit, his shied disappears, and you're free to hit him with all you've got. If you're close-range, make sure to watch out for his pellets.

For his second phase, he spawns in a dozen or so minions, and goes into rage mode. First, kill any minions nearby. Then, watch out for his attacks in this phase. He still drops bombs that you can chuck at him, but he also shoots out a set of chains hurt you if they hit, and locks off part of the battlefield. secondly, his pellet attack now has much more pellets, is faster and more random. You're best off hiding behind something for this one.

Lastly, some hint and tips. Watch out for the breakable floor, you can lose a lot of lives falling through it. I've personally had a great deal of success with Devil, but any will probably do.

EDIT: It seems that the Eradicator is actually not insanely powerful, I just managed to get a glitched version that insta-killed the bosses. I'd recommend picking it up if you find it, just in case you get the same glitch as me.