Well he's prolly the strongest character the SHs had to face up to this point, I'm curious how he'll react to the red hawk, will he be diminished, or will he recover like nothing happened? If it's the latter then obviously Luffy will have to resort to stronger techniques, maybe a red hawk gatling?
Except the Admirals and Zoro facing Mihawk
Power levels or tiers are not easily defined and very loose in One Piece considering that the abilities interact in a strange way sometimes (best example, Mr3 and Magellan in Impel Down), though this "problem" is less significant now with Haki being common, and the fact that conviction and determination is a core principle of One Piece and that also influences the outcome of fights.
Having said that, if i had to articulate it, i would guess that pretty much all pre-TS Shichibukai except Mihawk, who is the only one of the Shichis having tied with a Yonkou, even if it was in the past, and who probably will be Zoro´s final opponent, and Jinbe, who made it clear himself that he obviously has his strengths in water and not so much on land and whose nomination to the Shichibukai is motivated by political reasons besides maintaining the balance of the Three Powers, namely the friendly coexistence with Fishmen, are or were one level inferior to the highest level we know right now exists in the One Piece world, consisting of the Yonkou, the 3 pre-TS Admirals and probably a few equally powerful characters (if the new Admirals are that strong as well, we will see).
In Crocodile´s and Moria´s case, who both are Silver Medallists, it was made clear that they got weak over the years (both power-wise and mentally which brings us back to the determination thing) after their defeat against Whitebeard and Kaidou respectively but considering that they actually got that far, fought with those guys and were nominated for the Shichibukai for their strength, i think it is not too far-fetched to think that they were at one point as strong as someone like Doflamingo for example.
In the case of the other 3, namely DD, Kuma and Hancock, well, all three have extremely deadly abilities (deadly in terms of that they can decide a fight in an instant, whether it is actual death or in a different way) while Kuma has an extremely sturdy body and the other two have all 3 colors of Haki.
Obviously, DD is the strongest arc boss Luffy has had to face so far and he will be a real test before the Yonkou, though i still believe that ultimately, Luffy will beat DD by himself.