@red_thunder
Dude please read again, I said what you mentioned.
I dont have the legend you are using for blackbeard and garp

vs Garp, use your best point boosters and play AUTO. [...] vs Blackbeard, use your best point boosters
AND DREADNAUGHT SABRE ship.
Pretty straightforward:
use whoever you want. It doesn't matter if you don't have my same legends, any best multiplier characters you have can be used
.
also if you dont kill garp with your first wave of attacks and he ends up with low HP he will put 30 turn bind on your entire crew. I am playing NW I am not sure if you are playing on GL or something cause you didnt mention the bind thing at all.

Boss stage gives block orbs, 3 turn atk down (90%), 1 turn of Good barrier (2 good hits to break). On 1st turn vs Garp you deal very low damage if you happen to break the barrier but you shouldn't leave Garp on <20% HP especially later on.
I said implicitly why Garp puts 30 turns of despair because I supposed you knew what triggers that. Seems like you don't. BTW, I always play NW otherwise I wouldn't care.
As you see I mentioned such things.
The bind thing is where I ran into most of my issues, cause the 500x wasnt enough with who I had as captain (1.35 booster Kid).
That's the problem to begin with, teambuilding. That Kid only boosts STR characters. I guess you use other STR characters as crewmates? But that's not possible because except Kid all x1.35 F2P characters aren't STR. STR deals normal damage and the other characters aren't even boosted by his captain ability.
For Garp use the team I put as attachment if you have the units. Hawkins is the best as Captain because his Captain ability says:
Boosts ATK of Striker and Cerebral characters by 3.5x and makes PSY orbs beneficial for Striker and Cerebral characters.
All the other characters are Stiker and/or Cerebral so the ATK of them all will be boosted. If you need to remove despair Hawkins, Franky and Coby can do that
If you need any help send me a PM.