Champion Challenge was a limited-time event that featured a series of 15 and 20 stamina battles against "legends". Those battles are relatively easy, and had a decent chance to drop said legends books. Almost all the battles could be done with double Neptune/EXPPerona team with little difficulty, or Buggy+Neptune/EXPPerona.
The Champion Challenge was available from January 29 (19:00)–February 13 (18:59) PST 2018.
This Event is separated into 5 parts, with certain legend and non-legend skill books that drop from each one.
Available at Veteran and Expert difficulties. The bosses and mini-bosses of each quest will NOT drop wanted posters.
After having cleared each stage on each part at least once you will receive 1 rainbow gem for a total of up to 5 rainbow gems.
Part 1 Jan. 29 (19:00) - Feb. 01 (18:59) PST:
- White Knight Cavendish
- Heavenly Demon Doflamingo
- Trafalgar Law Warlord of the Sea
- Shanks Black Clad Emperor
- Portgas D. Ace Black Clad Division Commander
Part 2 Feb. 01 (19:00) - Feb. 04 (18:59) PST:
- Blackbeard Blackbeard Pirate Captain
- Sir Crocodile: Logia Ex-Warlord of the Sea
- Monkey D. Luffy Voyage Dream: Pirate King
- Donquixote Rocinante
- Kuzan
Part 3 Feb. 04 (19:00) - Feb. 10 (18:59) PST:
- Roronoa Zoro Straw Hat Pirates: Born Again
- Boa Hancock Warlord of the Sea
- Man-Eating Bartolomeo
- Captain Buggy
- Borsalino
Part 4 Feb. 07 (19:00) - Feb. 10 (18:59) PST:
- Issho
- Hawk Eyes Mihawk World's Strongest Swordsman
- Sabo Revolutionary Army
- Dark King Rayleigh
- Knight of the Sea Jinbe Ex-Royal Seven Warlords of the Sea
Part 5 Feb. 10 (19:00) - Feb. 13 (18:59) PST:
- Sakazuki
- Rob Lucci CP9's Strongest
- Sengoku the Buddha
- Whitebeard
- Marco the Phoenix Whitebeard 1st Div. Commander
For more details on who you could fight and what books would drop, see Reddit post here.