Lord Von Prong is the last boss you'll face before a dramatic conclusion and a hint at a possible sequel. He is located in the heart of Castle Von Prong which in this world rests in the top northeast part of this strange and exotic world. You'll definitely know who he since he has a massive head of hair and an extremely small donkey he rides on.
Probably a good idea to have a good supply of potions on hand before starting the fight. Kill his minions first. They can regenerate his health. He has a stun attack that can lasts long enough for him to kill you. It is entirely avoidable. You will notice him start to sing before pounding the ground causing his area of effect stun. When he starts to sing that particular tune, get out of range. As soon as he's done pounding the ground you can go right back in while you still see the musical notes swirling and start hitting him again. The only weapon that does any damage to him is the Prongenator 3000 you got from Eubrick. Don't waste your time with anything else. Just keep beating him with it until he's dead making sure you avoid getting stunned. If you die, you have to start the whole fight all over again.