Umm..its simple really. You create two paths (that are connected) and have them as long as possible. Put a level 1 turret at the end of one of them, forcing all the creatures to use the other one, while getting pelted by all your turrets. When they get near the end of that one, delete that level 1 turret you created blocking the other path, and create a new one blocking the path the mob almost reached the end of. They will be forced to walk all the way back, and go through the other path. Keep switching until all are dead.