If you're looking for the easiest way to complete those is just to use the Saints Crusader (the purple tank), just put it like 10 meters away from the icons where you start the instance and once it starts just leave the vehicle and go into the tank, while your buddy is driving you'll be able to use the unlimited and super powerful laser, you should then have no problem with this. Have fun =)
also: just be careful not to accidentally instakill any client or else you'll have to restart the instance...