Hi 
gameasher I made a quick example that might help you out. This sets enemy health as an instance variable. The pic below is the key to what you want just change the 20 damage to whatever your base damage is the *choose(1,1,1,100) Gives you a 1 in 4 chance or 25% chance at doing 100% damage. Hope this helps.
**EDIT** Actually I think my math is wrong since that multiplies it by 100 which isn't 100%....that would be base damage plus base damage again I think? so double? So really *choose(1,1,1,2). Basically change that last number to whatever you want the multiplier to be lol.
https://filetransfer.io/data-package/ORPFD05P#link