I got this in Chapter 8 after several tries. It all comes down to how high your attack is, what kind of enemy you're facing, and LUCK.

When I got this, I used Zoro's strongest skill attack- the Billionfold World Trichiliocosm (had to grab my copy of OP Manga volume 78 to make sure I spelled it right). This was at level 1, but it peeked at over 11K damage. The problem is... the amount of damage any of your Skill (and Normal) attacks do is random. When I popped this achievement, I had Zoro use this attack on 2 enemies that were weak against (IE, Zoro is a Technique type indicated by his green sword icon, so any enemy with a red fist icon, Power types, are weak to his attacks; this is shown by two red arrows pointing upward next to the circle of your enemies' faces in the left side of the screen during combat ONLY when using the correct selected character). Billionfold World Trichiliocosm is a group Skill attack, so you hit multiple enemies at once in a small area. When I used it on two Power types, only one of them was hit for 13000+ damage while the other was dealt much less.

The accessories you put on your characters also determine their attack. By Chapter 8, you should be able to fuse accessories to make them stronger AND you should have maxed out Robin's fusing skill so she can fuse up to four accessories into one. Just take a strong purple ATK accessory with up to four slots and fuse it with multiple grey ATK accessories. The one shop in the Ice Ruins sells +305 ATK Crowns, so you can stock up on them there.

NOTE: When you fuse accessories, there are 3 outcomes - Done, Success, and Triumph. Done is the normal outcome and you get the basic stats of the item you fuse into the base item, Success results in about 1.5x increase to the stat of the item, and Triumph results in 2x increase. Successes and Triumphs are random, however. I recommend you go to a tavern in Alabasta/Water 7/Dressrosa and make a save file first before you attempt to fuse accessories, so if you want to take a chance on maximizing your accessory fusions, you can Return To Title if you don't get any Successes or Triumphs and start again. Tedious, yes, but that's ONLY a suggestion.

Good hunting!

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