You go to you hack n slash folder in your steam folder.
For me that is over here: S:\Steam\steamapps\common\HacknSlash

Now you go to the folowing diretory.
\HacknSlash\Data\Scripts\Components
and you open the: "HealthComponent.lua"

And last but not least.
Changing the lua scirpt.
You have to change 2 words from false to true for it to work.
See the screenshot for what you have to change.
And then save, start the game and you should be invincible.