I tried the secret ending for five minutes and then gave up and looked it up. I had it almost right the first few times, then I started looking for secret doors.
I beat the game on all three difficulties, avoid the boss and get defense and agility up to 10, until you max out a sword, I picked chaos savior. Then, you should be able to overpower the boss, put remaining points into dexterity and strength. Easy, tedious, grindfest game. 4/5 stars.
I almost beat on normal, I avoided the boss to I got an upgraded weapon. Next, there was display bug, but I managed to somehow upgrade again despite showing I didn't have enough for a 2nd upgrade. I focused on defensive stats, strength might be the worst stat.
Seems you really have to understand the mechanics to beat the game on more difficult difficulties modes. What makes you gain experience and how much? Does the boss get more difficult if so what causes it to gain difficulty?
Seems to be no strategy in the beginning with a melee, put velocity to max and just raid a whole lot, collect money from castle, upgrade hero, rinse repeat until too slow and then start a new raid. Lots and lots of grinding at first at least.
 "Buccaneer!"){kind=icon}
 "MARDEK RPG: Chapter 2"){kind=icon}
 "Raid Heroes: Sword and Magic"){kind=icon}
 "A Grim Chase"){kind=icon}
 "The Savior"){kind=icon}
 "The Chamber"){kind=icon}
 "Obliterate Everything 3"){kind=icon}
 "Ode To Pixel Days"){kind=icon}
 "Pretentious Game 2"){kind=icon}
 "Can You Obliterate"){kind=icon}
 "A Castle For Trolls"){kind=icon}
You can get hints if you go to the pause screen!