I just reinstalled your game and checked the game. You did what I said not to do. PEMDAS (Parenthesis > Exponents > Multiplication > Addition) is important.
So, for example, you have a.atk * 0.8 - b.def when it should be (a.atk - b.def) * 0.8.
Let us look at
x * 0.8 - y
vs
(x - y) * 0.8
Using an extreme example of x = 100 and y = 90...
Standard damage: 100 - 90 = 10 (This is your standard damage)
100 * 0.8 - 90 = 80 - 90 = -10 (0 damage dealt or you heal the enemy depending on the engine and add-ons)
(100 - 90) * 0.8 = 10 * 0.8 = 8 (8 damage dealt)
So as you can see, they don't look that different, but they are very different.
As far as testing extensively goes... this is the free course. ;P