This Seeking Tech guide will explain how you can unlock the Synergy trophy and achievement in Clair Obscur: Expedition 33.
To get this, you need to meet the following requirements:
“With Maelle, use Percée on a Marked enemy while in Virtuose Stance.”
Use the following video as a reference:
This can be done by using early on by equipping the Marking Shots skill to Gustave and then using it to mark an enemy in free aim mode.
Afterwards, have Lune use Immolation to burn a different enemy. Maelle can then use Swift Stide on the Burning Enemy to switch to Virtuose Stance.
On the next Maelee turn, use Percee on the marked enemy to unlock this trophy.