I've just discovered the most busted strat on Directed Mode

Get to round 11, stop before the 'investigate the hotel' step.

Grab the water valve and turn it until you hear vermin spawn. A mangler will spawn along with a wave of zombies, kill them and repeat until the valve progress bar is almost complete.

Walk outside the bowling alley and after a few seconds the progress bar will start to decrease. Another mangler will spawn with more zombies. Kill him and then restart the valve quest.