In order to reset the world, you will need to get your last (20th) crusader Nate Dragon to level 150. This will unlock his final skill 'Reset the world', which costs 500Q gold.
You should be able to do this between areas 80-90.
A good formation to have once you unlock Nate is as follows (leftmost = bottom of screen and rightmost = top of screen)
Row 4: Natalie - Kaine - Panda - Artaxes
Row 3: Jim - Nate - Merci
Row 2: Khouri - Sasha
Row 1: Princess
Merci and Nate should be your main dpsers, but make sure you invest in Jim as well as he becomes very cost/dps efficient towards late game.
During boss fights, it's advisable to swap Panda with King Reginald as the King's five-second time stop ability is invaluable. Once you've used the time stop ability, swap the King with the Bush Whacker and whack away at the boss until he's beaten to a pulp.