Here is a quick house rule to use in your Dungeon Attack! games. When rolling the Boss Monster Dice, if a Skull/Defeated Monster symbol comes up on the first roll, you get a free re-roll in an attempt to defeat it. Another Skull on the re-roll and the Boss Monster Die is defeated. With this rule adventurers without a special ability to re-roll or who do not have a magical item, still have a chance to defeat the Boss Monster Die. Even with this rule each monster die may only be re-rolled once per turn.
Thank you Hector DeJesus for coming up with this simple, ingenious rule. We will be using this rule from now on in our games.