Beat any adventure in Shells Town using a crew of nothing but Slasher-class characters. Your Friend or Helper Captain must also be Slasher-class, or you may choose or not to select a Friend or Helper Captain.
Build your crew with only Slashers and clear the mission.
Have Cabin Boy Coby be Captain and beat Chapter 8 "Coby's Resolve" using crew of 6, including Friend or Helper Captain, with a total Cost of 24 or less. Your Friend or Helper Captain is included in the Cost.
Set Cabin Boy Coby as Captain of your crew. As subs choose characters with low cost like Evolvers or Boosters, alternatively use a single strong sub high levelled with high ATK.
Beat Chapter 7 "Destroy the Naval Base!" with a Saber Apprentice as Captain and a crew made up of Cannoneer, Pistol, Halberd, and Knuckle Apprentices. Your Friend or Helper Captain must be a Saber, Cannoneer, Pistol, Halberd, or Knuckle Apprentice.
Beat Chapter 3 "Son of the Naval Captain" with either Yosaku as Captain and Johnny as a Friend or Helper Captain, or with Johnny as Captain and Yosaku as a Friend or Helper Captain. Mission cannot be cleared if anyone else is in crew.
Try to obtain Johnny or Yosaku, then choose the character you do not have (if you have Johnny, choose Yosaku and vice versa) as Friend or Helper Captain. Do not fill your crew with subs, or the mission won't be cleared.