K, so this actually might be a bit complicated and I can't think of a title that isn't super long. Because all Yonkou/admirals are so close to each other in combat capabilities, it should only take a slight edge to win (example: Aokiji's laid back attitude vs. Akainu's ruthless nature is probably what created the opening for Aokiji to deliver the winning blow that melted Aokiji's leg).
So the question of the thread is what level of support would it take for a Yonkou level fighter to have an easy win? Based on the above mechanics, who is fighting who does not matter at all (haki is the equalizing factor anyway and all of these fighters have to have mastered haki ). Ah, just to clarify this essentially is asking for a 2v1, 3v1, 4v1, etc. to make an easy win for the team/group.
For me, one example would be Big Mom and Perospero. Based on Perospero not being able to use busoushoku adequately to safe his arm (context implies he had to pretty much depend on creating a candy shield), Perospero has zero physical capabilities that are helpful. However, because he has long range defensive and offensive support capabilities (I'm going to completely exclude the B.S. animated sea-slug that negates the ocean negation of fruit abilities since over half of it is submerged) he makes the perfect support. Big Mom can focus on attacking because Perospero can put up candy shields as needed for any big attack. Even if Perospero doesn't have the offensive busoushoku to cause damage, his ability to restrict movement is key. Perospero just has to create an opening so that Big Mom can deal the winning blow.
I'll give another example. If we magically give Luffy his awakening (ah, might have to see if these theories can be carried over) so that he is roughly Yonkou level (i.e. he has the adequate combat capabilities to deal a winning blow), it probably doesn't take much support for Luffy to win. No one on the Mugiwaras has both offense and defense without setbacks (I have to exclude Jinbei and all of his hax support with Gyojin Karate if a large body of water is close since everyone except Shanks gets weakened if they are over 50% submerged in water). Zoro can provide support with his Pound-Hou attacks, but he is limited to a fraction of his true slash strength/destructive power. Although it can be defensive to negate parts of attacks, it isn't a shield. Robin could provide excellent defensive support through Giganto Clutches (something that is possible theoretically but never shown since she has never produced more than two giant limbs). For example, she could stop or at least dramatically decrease the force behind a strike by having multiple giant limbs blocking it during the wind-up (rather than "Clutch"ing their neck, she does the same to the shoulder, elbow, wrist, etc. before a punch). Right now Robin does not have the speed to keep up, but if we magically give her this ability, the combination of Zoro and Robin's support means an easy win for Luffy.
The reason this is so important is because this negates the threat of Kaidou, Big Mom, and Blackbeard upon their defeat. The loss of the war will result in their crews breaking up (separate thread/theory on this elsewhere, but I guess the discussion on why this is likely how Oda handles the negation of future threats can go on here since I'm kinda merging multiple threads anyway). Once Luffy gets his awakening and Zoro/Jinbei go through their respective power-up from Wano Kuni, likely any combination of Luffy +1 should be an easy win (side tangent: I initially had Sanji here but if he couldn't KO Page One with his max attack in the Raid Suit, which is what the manga implied, Sanji is a tier below the other main fighters). This is why Shanks easily and rapidly defeated Kaidou during Marineford. The Red Hair crew likely has 10 people at Yonkou/admiral level. Any combination Shanks and one of his execs would be an easy win over Kaidou.
So I'll open it up to any character in the series. What would be the lowest support character needed for an easy win? I'm thinking answers are going to be all over the place.
Edit: Important Luffy caveat is that he is proficient with his awakening. Think of the scene of Luffy trying to use a Gomu Gomu no Pistol against Ace. The inefficiency is probably a huge disadvantage. So this does not apply to Kaidou vs. Luffy for this arc. It would apply to future Kaidou vs. Luffy fights if Kaidou were to try to intervene in the remaining years of the manga.