Watch a short video walkthrough of this stage.
Death Bringer, by far the most difficult boss in the game, sits on his throne. As soon as the two Skeletons enter from the right he gets up and attacks. Contrary to popular belief Death Bringer's Skeletons are not invincible, however they have twice as much health as Death Bringer so it's more likely you'll kill him first.
First off, use all your magic to weaken him. Jump attacks work well here, as they allow you to get clear of the Skeletons and hopefully lay a few blows on Death Bringer before they catch up.
Death Bringer knows all of Death Adder's attacks, but his lightning balls head in multiple directions and can be launched multiple times. He also has a dash attack.
The most crucial factor, however, is that every time he uses a normal attack on a player he casts a magic attack. He can use Gilius' level two magic, Ax Battler's level three magic, or even Tyris' level six magic. Unlike you, he never runs out of magic power.
After what seems like forever you'll knock Death Bringer's axe out of his hands and his Skeletons disappear.
You then emerge back into the main castle area where the King and Princess thank you. The map then shows your route. You then see the credits with Ax Battler and Tyris Flare resting while Gilius Thunderhead throws balls of text to a Silver Longmoan, who bats them with his club and makes them appear in the sky as credits.
After this the three characters face the player and throw their pots in the air, forming the letters E N D.