Again theres a very simple tecnique to unlock this in about 30 mins give or take, on your own
1) load up an easy realism dark carnival
2) continue playing until you get to the finale
3) leave the game (you must exit to main menu, do not call a vote as this doesnt work - thanks to DSmooove6 for confirming this) and start up a brand new realism lobby (believe it or not this works i was weiry about it myself) and set it to expert part 5
What to have before setting off the finale on part 5:
- nab a scoped rifle, a melee weapon, a pipe bomb for clearing out the horde when the chopper arives and an adrenaline for the run to the chopper
4) play on until you set up the finale, incap the 3 bots on the stage then set off the finale after grabbing the necesities you need: pipe bomb, adrenaline etc, then run up to the jesus rail
for those if you who dont know where this is, it is on the far left hand side of the stands when looking from the stage, by the blocked off entrance (Video below for visual aid), climb up to the top of the stands and sit on the rail, remembering to keep an eye out for infected climbing up beside you.
NOTE: its a good idea that the in the last couple of Special Infected you killed before setting the finale off was a smoker as he will then not spawn once you set it off, because he can sneak up behind you and ruin the achievement for you, if a smoker spawns when you set it off dont panic and try to find him with your scope before he reaches you
5) but anyway back to the achievement, when on the rail the majority of the common infected will glitch and not move (save a couple behind you on the roof tops KEEP AN EYE ON THEM, as if they fall of the roof they will be able to move and climb up to you. and specials (including the tanks) will not spawn after they have killed of the other survivors. But as soon as you get off the rail it will all revert to normal, so Just dont fall off or it untill the chopper arrives as they will glitch back and you'll have to die and restart from the part 5 saferoom
6) when the chopper arives pull out your pipe bomb and launch it across the map, also take an adrenaline shot or pills if you have them and run for the chopper, you should have enough time to reach it even if it arrives at the other side of the map
i managed to get this on my 1st playthrough, but my 3rd attempt at the finale, as the bloody single infected kept attacking me from behind! but i was very suprised that this method worked as it sounded stupid
NOTE: this method will work on other campaigns i just chose dark carnival as it had the most effective glitch spot, i believe there is a table in the hard rains burgar tank finale that has the same effect.
So just remember to always check behind you when on the rail as commons can still move down in the area below you but you should have no trouble killing them as they climb up!
hope this was helpful ;)