Note: always make backups! Use a text editor to edit the "LiquidGame.ini" file (in the \My Documents\My Games\Rise of Argonauts\Liquid Game\Config directory in XP).

Change the value of the MediumDifficultyDamageModifier line (or the line that corresponds to whatever difficulty you are playing) from 1.0 to 0.0. Save and start the game and you will be invincible.