I decided to use a RPG combat system. Although Bloodletting is card game similar to Pokemon and Magic the Gathering, it doesn't have the Energy cards which significantly influence the combat sequence and gameplay in those two games. Without the energy cards restraining the combat, it needs to be something different.
So I am using an RPG combat system. My character cards have Attack and Defense, which imitate and simplify RPG-type weapon attack (weapon damage and attack) and defense (Armor class and dexterity). The attacker "rolls" a 2 sided die as a save roll for his attack. If he rolls a number less than his Attack, then his attack is successful. The Defender rolls a 20-sided die for a save roll against his Defense. If he rolls a number less than his Defense, he has successfully defended (blocked/absorbed the damage) and he suffers no damage. If he does not successfully defend, then there is a damage roll equal to the roll of 1 6-sided die + the weapon damage multiplier on the card (1d +/- x). A Glock pistol has a weapon damage of 1d+3. I hope that this greatly simplifies combat, yet makes it interesting enough.