I played Dungeon Quest at Gen Con, and here is how it worked.
You can use Deathblows in additions to a Counterattack.
1. You both lay your hidden cards down and reveal.
2. Higher number wins, unless the loser has an appropriate counter attack.
3. Loser can play counter attack cards from their hand to raise ther attack value. All counter attack cards would be the same symbol, since it is rock/paper/scissors. If they can surpass your attack value, they now win. If it's a tie, all cards go to combat pile.
4. The winner of the round can do the Death Blow. This would be all cards from the combat stack of the orignal card played. So, if I played a ranged card intially, then I would grab all the ranged cards from the combat stack for a Death Blow. This blow may not kill the monster/player, but it could. Don't forget that all your counter attack cards count as one wound as well. This is why it is nice to play small counter attack cards first if you have multiples, to increase your total wounds to another character or monster.