Does anybody no how to do the last mission in left4dead

what i do is say near the weapons and when the tank comes tell the people who are playing to all gang up on it when it dies get back to the ammo then hide in the corner (hit any zombies away)then if you are low on health heal or heal a team mate is the health is lower than yours.when the army trck arives run to the car.get evryone back to the car to get the achevment no one leftbehind good luck!
if your still stuck add me via xboxlive;unikiller991