Level 3-2 in Streetpass Zombies (a.k.a. Battleground Z) has you inside a graveyard, where you have to protect its two gates from zombies. The gates are on opposite sides.
Even if I find several Miis with whom I can team up, and leave some of them posted by one gate while I defend the other one, they eventually give up and come back to me. As they take time to walk up to me, may get trapped behind an obstacle on the way and it also takes time for me to walk between gates, one of the gates remains undefended for several seconds when this happens.
In the 3rd wave of zombies both gates are attacked by very strong zombies and as a result whichever one is undefended goes down before I can reach the zombie attacking it, after the Miis I posted there leave.
Is there a trick to beating this stage? Is it possible to ensure that Miis I post somewhere do not abandon their post?