For this to work you have to manually hex edit the inventory file from your rage autosave as many know. The item you want to edit is the Adophis Infusion, which is a recipe that you get from completing the side quest Miracle Cure.
So Just make one of the items exit Wellsprings to generate a current autosave. Extract the inventory file like you normally would open with choice of hex editor and search for health. The string you want to edit is inventory/health/health_builder edit the quantity to 999 or 03e7 save and put everything back together. Load your save and voila. Just assign it to a quick use slot and spam away. I currently have 9960 health. Having no issues standing in front of machine gun turrets on Nightmare difficulty. You will die eventually lol but it takes A LOT of hits to even flash red. Hopefully they will include this item in an editor soon to make it easier but enjoy.
I used the procedure here
EDIT* It doesn't rollover currently at 12156 HP.