One thing I like about Tunnel Goons - there's no explicit support for PvP, which kinda discourages those distasteful shenanigans.
Nonetheless, there's often that person who insists on being a jerk "in service of the story." To support them, I propose the following:
Each player rolls 2d6 + skill + items as usual.
Each player's character has a DS = 7 + skill + items.
Each player deducts from their character's Health the total by which their roll falls short of the opponent's DS and by which their opponent's roll exceeds their own DS.
Example: a Brute 2 with a Big Sword and Chainmail (DS 11) goes up against a Skulker 1 with Throwing Knives (DS 9). Each player rolls. The Brute gets a total of 10. The Skulker gets a total of 9. The Skulker loses 2 (from their own roll) + 1 (from the Brute's roll) = -3 Health total.