I'm sorry but when was king Kong gun used against cracker??
That is not his strongest move. His full version canonball is his strongest.
Also I still don't understands why people r referencing the time frame between the two fights
Sure craackers fight with luffy was longer
But his fight with doffy luffy def fought harder and needed way more help against dofflamingo then he did cracker (no one busted up crackers organs) you also should factor in that luffy needed a break in his fight with doffy..
Lastly the only reason (until proven other wise) luffy was able to fight that long and contend with cracker that long was because he was constantly replenishing his energy/stamina with the food he was consuming... this was stated awhile back that he regains his stamina and will to fight after eating (which he had to do constantly to stay in the fight)
No, doflamingo had more help than luffy did. He sent a whole entire country against him and left to fight doffy in a weakened state, even Doflamingo patronized him for that while all doffy had was a stab from gamma knife. and he stitched his organs up so that's no excuse.
And just like you said luffy was eating and replenishing his stamina little by little against cracker but against Doflamingo he did not replenish his stamina at all. He came in the battle handicapped and worn out that he needed a 5 minute break from haki exhaustion. So his fight with Cracker was definitely a lot harder because he wasn't able to beat him easily even after replenishing bit of his stamina to use haki again.
Luffy said that Cracker has the strongest buso haki that he's ever come across, so Doffy can't even compete on that level.