I still can't believe people are relying on the fact that Rebecca had her helmet to support any claim of "luck" she apparently had for surviving Cavendish's/Hakuba's attack. If Rebecca was lucky to have her helmet during the D Block battle, Cavendish/Hakuba was lucky to have his sword, Durandal, during the D Block battle.
Like seriously, people were
allowed to bring equipment into the D Block before they entered the arena, given it did comply with the weight restriction and their weapon wasn't a firearm. Rebecca throughout the battles she fought most likely always wore her helmet because it matched the image of Kyros the statue was based around and as we all know Rebecca admires the legendary Kyros a lot.
Cavendish/Hakuba hit Rebecca's helmet because he wasn't fast enough to adjust his attack to hit anywhere else. The speed Rebecca and Cavendish/Hakuba have are seemingly close but due to Rebecca sensing the attack and moving her body in a position that forced Cavendish/Hakuba to hit her helmet which allowed her to in a sense parry his attack, it is Rebecca that seems to be the faster of the two. Cavendish/Hakuba wouldn't aim for Rebecca's helmet as his point/mark of attack, he hit the other contestants in their chest/stomach region, the only contestant which he failed to do so on was Rebecca was, who sensed his attacked, moved her body and used her helmet to parry his blow to negate the damage she would receive. The fact that Rebecca sensed she wouldn't completely evade Cavendish's/Hakuba's attack fully due to his speed closely matching her own and chose instead to use her helmet to parry it suggest that she is a VERY capable and adept fighter/gladiator - as illustrated by Sabo saying
she saw through Cavendish's/Hakuba's attack.
And as for the "fortunate" circumstance surrounding Cavendish/Hakuba falling asleep, well things happen like this in One Piece, it was Cavendish's/Hakuba's own fault for relying on an unstable ability during such a tournament. Until we know the exact mechanics behind how Hakuba is unleashed and disappears, we cannot conclude that it was only Rebecca's "luck" that she had won the D Block. But even if it was the fortunate circumstances surrounding Rebecca which led her to victory, this wouldn't be new to One Piece.
Luffy beat Crocodile because of the fortunate circumstances surrounding his first two defeats at Crocodile's hand. Luffy defeated Enel because of the fortunate circumstances surrounding his body composition. Luffy survived Lucci's attacks and was able to escape as the victor because of the fortunate circumstances surrounding the Going Merry's appearance. Luffy beat Moriah because of the fortunate circumstances surrounding the shadows Moriah captured. Luffy survived Magellan's poison because of the fortunate circumstances surrounding who he met. Luffy survived Marineford because of the fortunate circumstances of other characters protecting him (including his own brother). Luffy survived Hyouzou's and Caesar Clown's poison because of the fortunate circumstances surrounding his immunity to poison thanks to Magellan. And Luffy is going to defeat Doflamingo because of the fortunate circumstances surrounding Doflamingo pretty much screwing himself over with his arrogance and overconfidence.
Fortune or "luck" is an integral part to how battles are determined in One Piece. Rebecca winning because she was fortunate the circumstances were in her favour shouldn't come as surprise and shouldn't take anything away from her victory.