While Hyness is under his robe in his first form, he won't be all that tough of an enemy. He will have the Elemental Attacks of Fire, Ice and Electricity, but each are extremely easy to counter or dodge. You can even attack and completely destroy his purple projectiles. The real fight doesn't begin until he unveils his final form, where he will also zombify the three Mage Generals and force them to fight alongside him. The generals will have their Elemental Attacks increased during this fight, and Hyness will constantly be using them as projectiles. The worst part is when he uses them to create his own Friend Circle! When this happens, don't try to fight, just avoid his attacks until Hyness stops spinning around the arena.
If you can, definitely consider bringing the Sword Copy Ability into this battle, as well as three allies of three different Elemental Types, preferably Blizzard, Zap and Artist. If you combine your Sword with the Blizzard Element, you will leave behind damaging Ice when you strike. Should you combine your Sword with the Zap Element, you will also drop circles of electricity right on top of Hyness. The Artist Ally is merely to help keep your health in check during the epic battle.