Here's a written guide:
1. You will need to become evil, or good as i've been told, but I did this on the evil playthrough.
2. You will need to do quests in the three evil bases.
Check out the video on how to get to this point below!
3. After the third area, you will be tasked with going to see Robert in the bottom left of the map in the newly opened castle.
4. DO NOT TALK TO ROBERT. Go inside, to the far right, and break the wall.
5. Fight through the dungeon (not long) to the boss.
- The boss will do a rolling attack that you jump over, a ball bouncing attack that is not random, so it's fairly easy to recognize the pattern and step out of the way. There's also fireballs that rain from the ceiling that you need to find an opening, jump through, and let them fall through the floor. There's also a vomit attack when he turns green, and you just need to step back to avoid it.
- In between attacks, jump up, and attack his face.
Good luck!
How to get here can be found in the ultimate evil playthrough guide: