Never-to-do list:
1. Screaming "Fire in the hole!" in an airport.
2. Playing "toss the poop" in my own bathroom.
3. Apply to be the game creators' boss.
One of the easiest ways to beat the Stage 5 Boss is to max out the Defense style and equipping it before the boss fight. The Boss' attacks do not go through the shield.
Congratulations, you have taken down the city of Katanol. Your people shall be able to rest? in peace once again. Now, it's time you learn how to reload in-between fights.
Personally, Hard is the easiest as the Hero's skills are too overpowered; Poison + Tornado at level 3 Mana can easily take out enemy bosses faster than a mob of units.