give this a try.
1) start the no mercy finale
2) play as usual with friends or alone your choice of difficulty
3) once you summon the rescue vehicle, go into the jesus room
4) go into the left corner facing the doorway and go idle
5) after a few seconds this will then teleport you back to the safe room!
a friend and i basically just grabbed two sniper rifles and picked off any and all infected that came down the hall. you do get more kills while doing this alone. and since the a.i. cant make their way back, its unlimited kills for you! (however long you last). now you will get tanks and other specials but luckily you can do this on easy and save hours of frustration.
ALSO!: if you have trouble teleporting, try turning the flashlight off or looking down. if this has been patched in any way please let me know.