Here is a rather easy way to grab the Untouchable Achievement
From the menu go Instant Action then Duel and pick Deimos map, Now make sure the Bot Skill Level is on Godlike then set Goal Score to 20 and Time Limit to 60.
Once that is set hit X for Mutators and add in Low Gravity and Weapon Replacement and set all weapons to Impact Hammer
Now enter the game and head for the Orange Portal will takes you up to the turret and now you just got to sit there and take out the bot when you see it.
Video:
http://www.gamevee.com/viewVideo/Unreal_Tournament_3/XBox_36...