You need to defeat the Final Boss before you can challenge the Powerful Enemies. There are a total of 11 Powerful Enemies in the game. You need to defeat them all twice. The 2nd fight can prove a bit tedious, but you’ll be allright with the right preperation. All 11 fights are Boss Fights from the main story, just a lot harder.
I highly recommend that you buy white accessories from Yoisa in Dressrosa. Fuse your purple accessories with the white ones to get better stats. I focused on increasing my Attack and Defense. Increasing for example your Burn Resistance, will make the fight against Akainu almost laughable. If you fuse a characters accessories with 100%+ Burn Resistance, then he’ll only damage you 1 hitpoint each time.
Above and beyond all other consideration, abuse the food items that are made by Sanji. These can give you increased ATK and DEF and make the fights even easier. The same goes for the Trick Balls, abuse as many of them as you can.
After defeating the Final Boss, head over to Adio’s House and talk with Lim to unlock a new Side Story. Next you want to head to the Desert near Alubarna in Alabasta – here you’ll find a Yellow Cube (It’s located where you fight Smoker in the story). Defeat the Boss and head back to Lim, she will give you an item with the locations of the other 10 fights. Complete all 11 Boss Fights twice to unlock the trophy.
All 11 Yellow Cube Locations
Both Fights for Each Boss