Pick Axeman, upgrade Speed a bit, then focus on attack and health.
Before stage 2, keep doing training battles (before the last battle, you will notice a 93% and tough battle warning) until you get lots of Stones and money.
Upgrade everything you can the max you can and buy more stones with your money (waste it completely), do the same on stage 2 and there you go.
 "Flight"){kind=icon}
 "Cursed Dungeon"){kind=icon}
 "Onslaught2"){kind=icon}