A Glitch to your favour means this can be completed in a matter of 1 minute!
As I had already done 100% of main missions, I simply loaded a replay of a mission, then instead of completing it loaded another and the a third whilst in a lobby with 2 others. SO basically I'd start a mission, and then just load up a seperate one twice.
Every time it went up 33% and I got the achievement. I might have just got lucky, but certainly worth a try because if it doesn't work then its a matter of 30 seconds you've wasted,
I have my snap app open to check the % increase.