The easiest way to make a character invincible in a reliable manner, would be replacing the pre-made character with new ones made in the included editor

Inside the game folder core, find the file "entities_0.bos" and open it with either win rar or 7-zip

Locate entities/special/prefab and extract one of the 5 available .ent files, they correspond to the pre-made characters in game

Open the character file in the editor and change anything you want, including stats, skills, race and resistance

Just put in ridiculous high resistance in the base ability of the character, it should be under "Attributes-Derived"

After done save and replace the .ent file in the original compressed archive

Simply select that prefab character, and you can be anything you want, i usually use this to play as a robot, but you can also use it to play as an immortal human