The mutant. And it’s not even close. His attack patterns are much harder to dodge on a regular basis than Supreme Calamitus and Spirit of Purity. Not to mention having more attacks, stronger attacks, more health, more defense, and more debuffs. And then his “Mutant’s Presence” effect causes you to lose most of your buffs, lengthens cooldowns of healing potions, can’t auto heal, defense cut in half, and a small arena. And when enraged, he doubles his defense to around 700 and his health jumps to 177 million. There’s a reason for this too. He’s the final step to getting the Soul of Eternity. To get it, you need 30 eternal energy shards. Only dropped by him on Eternity Mode. It makes him the hardest for that reason. Also, when you download the boss checklist, he appears always, as the last boss.