GTA Online: Earn 25 platinum medals across Heist Setups and Finales.
In the Heist Update, platinum medals are rewarded to players who perform the best during a Heist Setup mission or Heist Finale. What constitutes performing above and beyond your crewmates are the number of kills and headshots one achieves, as well as accuracy and general contributions towards completing the main objective of the mission. After completing a Heist Setup mission or Heist Finale, you will be presented with the post-mission screen, which shows the players in the party and which tier medal they obtained: Bronze, Silver, Gold, and Platinum.
Generally, the easiest way to obtain this achievement is to partner up with a single other player and continuously replay Lester’s first heist, “The Fleeca Job,” which is the only two-player heist available. In order to replay this heist, pull up your in-game phone and give Lester a call. A drop down menu on the upper left side of your screen will appear, so go ahead and select the option "Replay Heist > The Fleeca Job." Now wait a few seconds and Lester will call you back, informing you that the heist is now available to start (indicated by a green "H" icon in your apartment's planning room).
During the first Setup mission here, the player going for the platinum medal should be the driver during the duration of the mission. As for the second Setup mission, the player going for the platinum medal should clear out most, if not all of the enemies around the Kuruma on the top level of the Del Perro parking garage, then proceed to drive the Kuruma to Lester’s warehouse. Lastly, during the Heist Finale, the player going for the platinum medal should be assigned the “Driver” role.
Following the method described above, one player can net three platinum medals in a matter of 20-25 minutes over the course of the three aforementioned missions. As mentioned, you can continue to replay this heist multiple times, so grinding out the achievement on “The Fleeca Job” is by far the quickest way to obtain 25 platinum medals. Just take note that there is a 90-120 IRL cool down to replay this heist, which can prove to be a burden. To bypass this cool down period, please refer to THIS POST.
If for whatever reason you lose track of your progress for this achievement, you can check how many platinum medals you currently have by following these steps: PAUSE > STATS > AWARDS > HEISTS > DECORATED AWARD. This award tracks how many platinum medals you have achieved. Thanks to Colonel Gray for this tip!