Do it as a boss reward. Floor 50 boss drops rotate token or something like that. Also, good idea for bosses is to make them have a unique skill. Have like three skills, but as the floors go on, the skill increases in potency. for example, floor 10, 10% more slashing damage. 20, piercing. 30, blunt. 40, fire, 50, ice, 60, thunder. starting on floor 70, increase potency to 20%, or something along those lines. Also, doesn't have to be in that order specifically.