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Step 1: Invite a friend & Start up a Hardcore Gamemode I recommend domination.
Step 2: Find a spot on the map where you and your friend can hide.
Step 3: Put your riot Shield on your back and pull out your secondary.
Step 4: Have your friend get behind you and shoot you in the riot shield as long as you have your secondary out it will rank it up.
Notes: I think this will only work with the absorb so much damage challenge.