For this achievement, you’ll have to execute at least 150 Synchro Rushes.
Synchro Rushes are a new mechanic added to Monster Hunter Stories 3: Twisted Reflection that are similar to an all-out attack for fans of the Persona series.
You’ll notice that when fighting a monster, there’s a secondary bar under the monster’s health bar made of diamonds. As you do damage to the monster, these diamonds will deplete, and once they’re all gone, the monster will topple (in slow motion!) You’ll have a window of opportunity to hit the Synchro Rush button and perform an all-out attack, with a cool sequence of everyone in your party doing their bit.
Once you’ve successfully executed 150 Synchro Rushes, the achievement is yours!
Like many of these achievements, you can check your progress in your Ranger Records by pausing the game, navigating to “Ranger Files” (one tab to the right), and then selecting the Ranger Records dossier (the red one on the right). The Coordination Shield medal is the fifth medal on the second row.