Every 1 second
Pick each unit in [unit group] (Zombies) and do action:
Issue order to picked unit: Attack to point: position of unit: closest unit to picked unit in Unit group (Humans)
So every 1 second, each zombie will attack-move targeting the position of the closest unit that is not a zombie. Note that this considers absolute distance, so a unit that is up a ramp or behind a wall will still register as closest even if the essential walk distance doesn't make it necessarily so.