If you have no one to play with I recommend Mission 2. I did this alone with a second controller on Poozer difficulty after finishing the game on EK. At level 10 damage taken was minimal, but if you feel one player is more at risk just switch controllers, I found that the bad guys aimed for the player not moving (i.e. the controller sitting on my ottoman while I was playing the other one). Even when navigating through the laser grid in the tunnels it damage taken was practically nothing. An easy 5 minutes of on rails shooting and now you have 20 points and the only multiplayer achievement of the game done.