Here is how I boosted this with two controllers:
Map:Boardwalk (doesn't matter, just one you are familiar with)
Gametype: Swat (in Slayer category)
General settings: turn off score and time limits
Player Traits: Damage Resistance to 10% (don't know if this even helped since it was Swat)
Weapons and Damage: Ability Usage to Unlimited
Movement: Player Speed to 50% (So I can aim quick enough)
Sign on a guest account and set their button mapping to RECON, so that you can use the sprint (LB) and the left stick to run to your main account.
With your main, pop them in the head to get a HEADCASE medal. These are what you are looking for.
Takes 5 mins or less!