Use a text editor to find and edit the user.ini file in the system folder in the DX2 game directory.

Locate and replace the following lines:
[Difficulty]
; AI Damage multiplier - damage AIs take from player
Difficulty_AI_Damage_Easy__d=1.25
Difficulty_AI_Damage_Normal__d=1.0
Difficulty_AI_Damage_Hard__d=0.75
Difficulty_AI_Damage_Real__d=1.5

Replace all the numerical values with values greater than 4.0 and a maximum of 9.0

God Mode Hack:
This involves editing a game file so create a backup before proceeding.

Use a text editor to locate and edit user.ini in the system folder in the DX2 game directory. If the user.ini is read-only, discard its write-protection first before editting.

Locate the following lines:

; Player Damage multiplier - damage player takes from AIs
Difficulty_Player_Damage_Easy__d=1.0
Difficulty_Player_Damage_Normal__d=1.0
Difficulty_Player_Damage_Hard__d=1.25
Difficulty_Player_Damage_Real__d=1.75

Replace all the numerical values with 0.0 (you always need to include both the tenths place and ones place digits at the minimum) and you take no damage from enemy fire.