Frey is your first boss encounter, she wil fight you on the 10th floor of the first dungeon. The fight can seem quite intimidating at first, you are in a big room with quite a few enemies and Frey herself can pack quite a punch.
The easiest way I found to beat her is to make good use of the environment and clearing out the other enemies first before going for Frey, otherwise you will easily get trapped in a corner and wiped out. In the top left and right of the map are pillars that either seem to spawn more enemies or make helpful items disappear, make sure to take them out. If anyone can provide me with an exact description of what they do, I will update this solution.
Equip Fangs that allow you to create a distance between you and your enemies, such as the Harpy's fang (attacks and let's you jump 3 squares backwards), the Dvergr's Fang (attacks and knocks the opponent 10 squares away) or the Warrior Gobling's Fang (attacked and knocks the opponent 10 squares away). The latter is definitely advised to also reduce the chance of traps messing up the fight. The Aloe Slime's Fang can be useful for healing if you don't have a lot of potions
Be sure to have all of your abilities primed by raising your Brave before this encounter as you want to be able to use them when you're in a pickle.
In the room are a few crystals, smash these to easily raise the readiness of your Fang's abilities when they become unavailable. Also be sure to pick up the potions as you kite through the room and use them when you need to. Other damaging items can be very useful too and something like a Blunt staff can slow enemies, helpful for kiting and picking off the enemies surely but slowly.
When Frey uses her skill, she becomes confused and her attacks get stronger. Keep your distance in this phase as she can hit quite hard. This fight is not easy per se, but as long as you can manage what's stated above and have a solid inventory of Fang's and recovery items, you should be okay.
For me, the achievement didn't pop until I visited the next floor and finished the dialogue with Gosume.
Any feedback or suggestions are always welcome, so please let me know if there's anything I should change on this solution.