I'd like to know your reasons for thinking this. Just curious.
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Imo Doflamingo is the bridge between upper high tiers and top tiers. He is above vice-admiral level while being weaker than admiral level, capable to give an admiral level fighter a mid difficulty fight before losing. Luffy will defeat Doflamingo, maybe with plot help or not, but he'll at least prove that he can defeat a plot restricted Doflamingo, which would put him in the range of at the very least pushing a healthy Doflamingo to high difficulty and thus i see a serious Luffy capable to push an admiral in the range between higher end of low difficulty and mid end of medium difficulty.
Zoro shouldn't be much weaker than Luffy at this point, so i think he can push an admiral to the higher end of low difficulty especially when on the contrary of Luffy, he is more serious in fights. Sanji should probably push an admiral between the lower end of low difficulty and the medium range of low difficulty. Combining their powers, i see them giving an admiral a difficulty ranging between higher end of medium difficulty fight and mid end of high difficulty, especially when you know that when they attack together, they are stronger. I also consider MF Garp to be slightly weaker than an admiral, especially in the long run and because of this, the difficulty in my opinion will be between medium end of high difficulty and above the high difficulty range