EASY CO-OP SOLUTION!!
Go down to the Rocket launch pad together, buy the trip mine off the wall, one person throw it down near wundersphere and let the other player train zombies, once they are following the player outside the wundersphere, the player about to portal will have a trip mine on the ground near the wundersphere or anywhere really, wait til the player training the zombies is close to the Mine and activate the wundersphere, the player with zombies must drag the zombies towards the trip mine and BOOM, easy achievement..
Rinse and repeat using other player