*Edited*
simo949 suggest that 2V2 Gnasher is the best way to do it (7 rounds to win) it's like 7-10 minutes per Match.
*Original Solution*
I can confirm that playing against bots doesn't count, you have to play quick match, i assume that rankeds count too
I got it playing warzone (4 rounds to win)
Happy hunting, gears!