Note: This cheat involves editing a game file. Always make a backup before proceeding.

The file you will be editing is easy.var (for Easy difficulty, use other files for other difficulty settings). It can be found in the "25 to life\data\var" folder. First, right click the file and select properties. Uncheck "read-only" and click OK. Open the file with Notepad or Wordpad. Locate the line that reads "AI2PlayerDamage 0.14" and change 0.14 to 0. Save the file. Now when you play you will take no damage from AI characters. You can also adjust to a higher number if you want to allow yourself to take some damage.