I guess we can avoid the problem of one shot kill by implementing a simple rule for the boss.
In White Dwarf 159 you can find "Rivers of Blood", an Advanced Heroquest adventure by Carl Sargent.
It introduces a Vampire boss and a special rule of Blood Points, but I'd prefer to call them Chaos Tokens (not to be confused with the chaos tokens from the Ogre pack).
Also an Evil Wizard deck can have similar features. It can be applied to almost all the villains or mid-bosses; the stronger he is, the more Chaos Tokens he will start with. I believe the Witch Lord deserves it!
And here is how the Blood Points work:
White Dwarf 159 - Rivers of Blood wrote:Blood Points
This is a unique rule which only applies to Count Von Bleistift. The Count is a very, very old Vampire, and much more powerful than any ordinary undead monster.
Within the sanctuary of his own catacombs, he is able to draw upon ancient reserves of strenght and power. To represent this, the count has a number of "Blood Points" which he can spend on the following special actions (only 1 special action per exploration or combat turn):
1BP Move at double normal rate for one turn.
1BP To negate the effect of any one hand to hand blow, or missile, which struck him during combat turn.
1BP To completely negate the effects of any one spell cast at him during a combat turn.
1BP Reset any magic trap, anywhere in the entire dungeon, which the Heroes have managed to disarm.
2BP Teleport from any one location within the dungeon to another.
The Count starts the adventure with 6 Blood Points.
He can increase this number by successfully biting a Hero...