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.
PLEASE DO NOT ASK SOMEONE TO FRIEND YOU SO YOU CAN COMPLETE THIS OR ANOTHER MISSION. If you need Yosaku or his pal Johnny to clear the missions, don't bother posting here. Even if someone eventually reads your message and adds you as a friend, it may be weeks after you asked. And then he would have to keep the character you want as a main for who knows how long. NOBODY WILL DO IT! What will realistically work:
You'll just get lucky, and one of your in-game friends will put whoever you need as a main captain without coordinating with you (because in game coordination is impossible)
Find a real-life friend, or at least someone on OPTC wikia discord (check reddit for links and ask there if you cannot find it) and coordinate in them in real time (you know, by talking face to face, or chatting on discord or such).
start an alt account, keep making FP pulls, and farm the RSRF FN when it comes around, and finish this all by yourself.