How to level up attack and defence in game?

In my game, player can increase attack or defence when level up. If player select defence increase each time, finally player defence will be larger than monster attack.

So, damage = attack - defence do not work, player take no damage in game is just a bug.

I want the damage increase as an exponential function, y = a ln (x) + b, but I don't know how to deal with defence to avoid "take no damage" situation.

 

Instead of subtracting defense from attack, how about have defense reduce damage as a percentage? Then you can cap defense at a high percentage so as to never reduce by 100%. Maybe cap at 0.9 or 0.99 or so.

Code (csharp):

1. float damage = attack * (1f - defense);