This achievement may seem like it'd take longer than it actually does, but honestly, you can probably get it in the first hour of playing the game.
All you really need to do is complete the tutorial/quests in the top right corner. If you still don't have Power Rank - 100, then complete the dungeons scattered around the place. There will be a tutorial showing exactly how to locate and complete them. I suggest making your way to that quest to learn for your self how the game wants you to learn.
Equipping items to your character may also increase or decrease your power rank depending on what you previously had equipped in that slot. Each equipment/item shows how much power rank it adds to your character. Equip whatever adds more power rank.
The earlier dungeons are quite simple and can be completed solo. Remember
are both attack moves as well. Once you kill all the enemies in a dungeon, you will be rewarded with some bonus XP as well as a chest at the end.
Technically this achievement is story related since you cannot progress further into the game until you reach a certain Power Rank.