How to script a damage counter into a leaderboard?

I'm in a need for a damage counter which would add up in a leaderstats, I'm a complete noob in scripting, I know very little about leaderstats or how to count damage taken by (for example) a sword.

Clear explanation:

A normal Humanoid has the health of 100 (Health Points), the Damage counted would be how much health has been taken by me from another player in a fight. This would add up in a leaderstat next to WO's and KO's in the game.

If anyone is up for helping me in scripting something like this, and it wouldn't be such a big challenge to them, or they have a ready script, please help me out here.