I refer to the Spirit Riders in RofTWL quest 2 "The Cold Halls"
"The Spirit Riders are hard to kill and so, each time they fail to defend against an attack they must roll one attack die. If a shield of any kind is rolled, the Rider will die, but if a skull is rolled the Rider is unharmed"
I love these guys when playing Morcar, they wind Puck up something chronic. I limit the number to the tombs available (i.e. 4) in any quest and alway place the tomb in the quest and use the tomb treasure cards when they are searched. If every monster was to use the "survival" die throw (as we call it) then I think that the quest would never end. Haveing a special monster with either high attack and defence dice or some of the more powerfull Black Magic spells available also allows for some epic battles, and balances the Monster/Hero equation.
Some of the 1 B/P monsters do get through the Heros defence, so putting some of the above later in the quests does mean that the Monsters can win. ( Ask puck, first quest I wrote I killed all of his Hero's with just 2 Spirit Riders and they were all armoured up to the hilt.

)
Sometimes it is just the luck of the dice.