@Maju:
because they just regenerate if he just punch them,so cracket stamina would never deplete,it is stated right in this chapter
Yes, but they wouldnt stop Luffy's punches because they would be too weak. So reform them all he wants, Luffy should shred them and then tear into Cracker.
@Kaido:
Would that really have been interesting though? Typically one of the main components of Luffy fights is their ability to remain serious and be fun at the same time. Let's not forget that Luffy's battle against Crocodile was essentially an inversion of this fight but with Luffy holding water instead of food. Then there's Luffy shutting off his mind against Enel and the ridiculous-looking Boundman form that absolutely destroyed Doflamingo.
It's not that Luffy isn't taking the fights seriously, he does, but he cares about winning, not about appearances. He exploited a clear weakness in Cracker's abilities that only he would think of, and went to the limit to completely overpower Cracker. And what do we have at the end? We have Cracker, completely fucked up, probably headed straight for the Whole Cake Chateau.
The prior fights you mentioned Luffy had to come up with a way around the enemies abilities, so he was creative. The fight against Cracker he had already gotten around the hardness of the biscuits so he shouldnt have needed to eat them.
The fight could have been interesting with Luffy fighting and struggling and then somehow figuring out the weakness. Maybe a scene where he thinks "these are biscuits, i should eat one". So he picks a piece up off hte ground and its super hard but as he salivates on it it noticeably softens. So he asks Nami to make rain and starts his counter attack or maybe have Nami be the one that notices this weakness.