Open the breed folder, then go into the run folder, then the scripts folder, then find and open the file named difficulty.txt
Under the lines saying Damage0 and Damage1 are numbers such as 0.17.
The ones under Damage0 are the amount of damage you take, reduce all of these to 0.0 and you will take no damage other than falling or collision damage.
Under Damage1 are the enemy damage modifiers, change these from 1 to 10 or even 100 for 10 to 100 times the damage.
You can also type in -1.0 for the numbers under Damage0 and enemy blasts will actually heal you!.