Fixing both Leech and Combat kid would be as simple as making Leech heal only when you deal damage, maybe you heal X% of your damage you deal with autoattacks. This increases your longevity without heals, but you can't just soak up your own blood to heal for more than you're taking in damage. It also means you'd only get heals on your turn, so comeback kid wouldn't be able to keep you alive through several attacks.
I like this solution.
Problem is, it would need a rework of the game system.
