This achievement isn't quite as hard as you might think. You don't have to finish the last gen per say, just any gen after the third survivor dies but before the killer can close the hatch and you can accomplish this by getting a gen to 99 or saving a Brand New Part and trying to rush the gen when your last team mate is being chased. You could also SWF to coordinate that bit. If you manage that, then wait for the killer to close the hatch and leave through an exit gate and you'll get it. I recommend Wake Up and Spine Chill as they'll help you open the gate just a bit faster under pressure but those few seconds can be enough.