A Hole New World

While I am still uncertain as to what actually triggers this guy to appear after the regular final boss (Baduk), you can be sure he'll appear on a 100% run, which you have to do anyway. Regardless, this achievement, contrary to its description, is for beating Super Baduk with no damage taken.

This boss is actually really easy, the only annoying part is that you have to beat Baduk again every time you want to fight him. Before you touch the black gem, switch to the blood potion, and as soon as you have control, run right and throw a few attacks to destroy his... eye? Then switch to fire-ice and run left. You now have to take out the orange gem on his hand, so just stand under it and throw balls up at it when it's resting. Run to the right if he slams it down and you'll be safe. I never saw the blue hand move during this phase, but if it does attack, jump to the ledge on the left to be safe. Next, run left and drop down to the reverse screen, then quickly run right and throw a lightning potion to break the gem here before the big tentacle things hit you. Then flip back up and stand on the left ledge, where you are safe from the blue hand's slam attack. Just throw fire-ice potions until eventually one bounces into the blue gem on the hand. Now immediately fire a charged Fäy attack at the boss' stomach, then another one when it starts glowing to finish him off. Ezpz.