One thing that works pretty well to mitigate that effect in Dungeon Hero, which is based on the mechanics of Fable on Your Table, is to allow the player to spend a point of Stamina to force the re-roll of any die in the contest. You can do this as many times as you need to as long as you can pay for it with Stamina.
Maybe I should add that as a general rule; that would help "even out" those particularly damaging rolls, and add another strategic element - do I burn stamina to boost damage, or should I keep some in reserve in case I suffer a catastrophic roll?