If you don't mind the short wait between replays, you can replay the first heist, the Fleeca Job, with a friend over and over.
Simply host the heist replay yourself and invite your friend it, and upon completion, have your friend host the heist and invite you in.
Each player can only host the same heist replay once, and then they have to wait an hour, real time, before they can host it again, so taking it in turns with your friend to host the heist replay allows you to get up to 6 Platinum medals every hour (3 missions, 2 people hosting).
To gain the platinum medal, do the following for each set up mission and the finale:
1st Setup Mission (Scope Out): Make sure you drive, the driver always receives the Platinum medal.
2nd Setup Mission (Karuma): Drive the Karuma, but make sure to also kill a decent amount of the enemies, as if your friend kills enough, s/he can steal the platinum medal from you.
Finale: Be the driver, the driver always receives the Platinum Medal.
Do this 4 times and you'll only need one more platinum medal for the achievement.