In order to unlock the Heavy challenges, you will need to earn at least a Bronze medal on all of the challenges prior to this one. The only weapon available for all three is the Minigun. These challenges are slightly different than the previous ones, since it doesn't follow the normal rules. You will always get 100 points for hitting a target, but as you hit them consecutively, the multiplier will increase (maximum of 3x, so 300 points).
These challenges can be among the easiest, since there is no penalty for missing and you do not need to reload at all. Also, considering you can keep the trigger held down, it's can be very difficult to drop the multiplier.