You will find It, the Prince of Annihilation at the bottom of the final layer, the Fleshsore layer. The fight is fairly simple if you can avoid his two insta-kill attacks. 1) The expanding eyeball which will explode, 2) The vertical laser moving to and from the boss.
The expanding eyeball attack can be avoided by constantly moving, and double-jumping (or flying if you're a Necromancer). The laser attack can be threatening, however on each side of the boss chamber, there are a few blocks you can hide underneath, or on top of, to avoid the laser attack.
I personally was not strong enough by the time I reached the boss whilst in Normal mode, so I decided to swap the world to Nightmare mode in order to craft better armor, find a better weapon, and level up.
Once I had done all this, the boss was super simple, as long as you can avoid those insta-kills.
Please see the below video on how to unlock this achievement: