To add on to Musell, when creating your pathways, make your pathway to the exit as short as possible with single side paths leading to other minions and treasure. This allows a much better chance at lower level heroes to catch up by increasing the chance that they will kill an enemy the higher level hero might have missed. You don't need to have your main hero to attack all available enemies to level him up. They level up after killing only 1 or 2 enemies that are 1 or 2 levels above it. Also your heroes won't enter a level 3 levels above it's current level in case you're wondering. With this stradegy, Not only did I beat it after 20 days, but I had 4 heroes at battling the boss. (as for the boss, create lots of side pathways and minimize the amount of time the heroes are on the dragon. This gives them time to heal up more so they last longer battling it.
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