It's unfortunate you can't prove that statement..
Luffy > Doflamingo
KKG says "hi"
GG
One King Kong Gun taking out Doflamingo is flawed. You say that because 100% of the time that Doflamingo has encountered KKG he's lost to it, he will always be knocked out when hit by it, even at full power.
By this same logic, 100% of the time Zoro has encountered the Yeti Cool Brothers, he's lost to them. Therefore, he would lose to them right now in one shot, because that's what's happened 100% of the time. 100% of the time he and Sanji have encountered the bomb of the Yeti Cool Brothers, it knocked them out cold in one hit. Therefore, the bomb of the Yeti Cool Brothers>Sanji and Zoro 100% of the time.
100% of the time Zoro has encountered Rob Lucci, he was defeated in one move. Therefore, Zoro will always be one-shotted by Rob Lucci.
See how retarded it sounds to call one instance of something happening "100% of the time" and then saying that's what will happen 100% of the time?
Not to mention that KKG drained Luffy of the Haki he had to go Gear Fourth, implying that it uses the same amount of energy as the rest of the Gear Fourth assault. And while that means KKG has an obscene and tremendous amount of power, it also indicates that one KKG isn't enough to defeat Doflamingo because the assault that takes up the same amount of energy as KKG failed to do so.