Some script tweaks to Sergeant Mark IV's Brutal Doom mod where you can avoid all damage without sticking to cover if you try hard enough! Disengage if you start getting heart problems.
This isn't the mod. Stuntman and Daredevil are two additional difficulty modes, adding up to five in total. Stuntman is easier than normal, and Daredevil is less than half as difficult as Impossible. Instructions are on the main mod's summary / description.