This can be done with a Second account & second controller on a LOCAL match or online with a boosting partner.
Set the gametype to Submission, so you won't need a third player as the meatflag is provided for you.
Set bots to Zero
Customize all weapons to Boomshot.
Have your primary account capture the meatflag while your secondary or partner (if done online) shoots the meatflag with the boomshot.
Choose a nice small map like Security & it shouldn't take long.