When you're in the mansion, go to the second story. Then head all the way to the left side (point of view when you come up the stairs). You will see the last pole that stands before the wall.

Camp outside of the banister but stay against the pole. Your team will cover your back and zombies will climb underneath you and get stuck beneath your feet.

Melee them once and they will stand in midair. Melee them twice and they will fall.

You can wait there until the boat arrives.

P.S. watch out for tanks and the ruble they throw, they can possibly knock you off.