Here's a link that will tell you everything you need to know about your Power Rank and what is the best route to level it up.http://trove.wikia.com/wiki/Power_Rank
This achievement with the rest of the "Power Rank" achievements are story related as it's required to progress through the game and enter new portals.
I will add a few tips that may help you along the way if you don't wish to view the link above.
Also keep in mind, upgrading your items at the forge can boost your Power Rank even further. As well as upgrading your Gems.
Completing dungeons is the way to go for this achievement. You want to kill everything in the dungeon and grab all the loot. The better the loot, the more power rank it will add to your character. Replace your items for higher power rank items. Leveling up your character also adds 15+ power rank. You also can get 2+ Power Rank for every Mastery Level.
I also included a video below if you'd rather watch that.Video Credit
- Scythe Plays