OCTOPATH TRAVELER

Ophilia’s first chapter takes around an hour like the others. The achievement pops after the cutscenes in town when you defeat the Guardian. It’s weak to ice and wind. When it’s health is about half down it summons three Dark Wisps that are weak to fire and light. They won’t attack but do a three turn count down and explode, defeating themselves and a lot of damage to your whole party.

I found it helpful for Ophilia to Guide the town person on the far east side of town just above the exit - kind of hidden between buildings. When Ophilia chooses Summon in battle, he casts a fire spell that damages all enemies a bit and helps break the dark wisps.