Became Able to Use Each Element’s Strongest Art at the Same Time
You need to be able to cast the highest powered Art from each element with someone in your party, but not all with the same character. These Arts are the ones that require 20+ of one EV and 10+ of another, and cost around 400 EP to cast.
The best moment to do this is in the final dungeon, just before the final boss, when you have all the high-level Quartz and at least some of your Master Quartz are maxed out.
When you’re in your ‘base’ The ship all characters count as ‘in your team’, so you can easily get one of these Arts with each character until you have them all.