Metaphor: ReFantazio

When you head into the Mausoleum on the 11th of June, you’ll fight against an Undead boss (no names as it’s spoilers), and before said fight, Hulkenberg will unlock her Archetype and join the fight alongside you.

This event will also unlock a new mechanic too, called Synthesis skills, which are available to use when you have two corresponding Archetypes out on the battlefield i.e. a Knight and a Warrior. If you have those two on the field, you’ll be able to get Hulkenberg and Strohl to perform special moves (which takes up 2 turns if you don’t hit a weakness/critical).

So, when you’ve done that, as either Strohl or Hulkenberg (whoever goes first), hit the button, which will bring up a new menu. If it’s Hulkenberg you can use Jump Thrust or Magic Guard; and if you’re Strohl, you can use Round Slash (which does weak physical Slash damage to all enemies for 14MP).

Use either of the above (or any Synthesis skills, for that matter) and you’ll unlock the ‘United Front’ achievement.