Ok so step by step, find the opponents exact health (search for original, shoot him once fuzzy for less) it'll be a value that is repeated, find the opponents health and single it out (search for it so the rest of the results are gone) i.e. Opponent has 637 health starting, you shoot him it's 521.38246 (write this number down exactly)
once you have his exact health just write it in either calculator or on a piece of paper. Shoot them, subtract the new health after you hit them again from the number you had just wrote down/put into a calculator. I.e. Shoot him one more time, his health drops to 404.83926, so then you subtract it from the original. I.e. 521.38246-404.83926=your true damage.
Once you have that "true damage" you can then search for it exactly in IGG at the beginning of each battle (turn float in the More section of IGG to a small number such as .001 to reduce # of results)
this way it's one step at the beginning of each fight rather then having to shoot the opponent and search for his health each time.
Note: your true damage will stay the same until you start changing parts, so keep in mind you'll have to find it again if you upgrade weapon.
Hope this helped!! Let me know if you're still struggling