Here's a more detailed method that worked for me. It's actually a palm to forhead moment. Played on Easy/
Exit Safehouse and move down the hill. Make right and enter building with generator on second floor. Turn it on. Turn around and jump down and exit building moving toward generator near bridge. Kill infected horde as you move.
Turn on 2nd generator and start moving toward last generator down the hill in corner near scaffolding killing the horde as you go. Turn 3rd gen on and move to bridge. by the time you get there the barrier will be down.
Make sure everyone is on bridge. Climb up ladder and hit bridge switch. Now quickly climb down ladder and jump off raised bridge. Don't hesitate. Quickly move to generator near bridge taking out the few infected remaining from last attack.
Two of my friends and I tried to get this achievement, one of them was Bill and he sacrificed himself, the rest of us stayed on the bridge but didn't get the achievement, is Valve having problems with some of their achievements?