if you make the path to the exit short and make sure to have some enemies on some side paths it increases the chance that the lower level adventurers catch up to the higher level adventurers by sometimes attacking the enemies higher level adventurers miss. This is the method I use to get the hard badge. I also make sure to only put 1 money bag down directly in front of the exit or as close as possible so the adventurers don't keep hitting it and wasting your money. Finally, always sell all armor you pick up to unlock new levels faster. For me though, I typically find rogues best as they aren't injured by traps, important on later levels.
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