Please tell us what game it is for, the name of the achievment, and what you must do to get it.
Just recently I got Untouchables in Left 4 Dead ( No survivors take damage after you contact the rescue vehicle. ) I got it without using the Jesus room or going on top of the plane with friends. I simply waited in the far corner of Dead Air's Runway Finale close to the airport. Just held out and killed all the zombies as they aproached.