A lot of Robin Fans want a One on One. Why?
I guess a one on one would be nice - I can admit that - but it really isn't necessary for her character. Her role is not fighter, and the manga is not all about fighting. She can hold her own in a fight, yes, and seeing some abilities showcased would be fine, but since she is not primarily a fighter, we should see her character progression primarily outside of fights. Robin is one of the best "support" characters around. She is highly mobile, and she is intelligent. She travel with ease, can eavesdrop, can figure things out, can read poneglyphs… Her character does not fail if she does not fight - she usually doesn't need to.
Second, Rebecca. I don't like Rebecca, but I think people are also giving her too much credit. People are pointing out that she is a gladiator focused on defense, and specifically dodging; why would she need protection, people ask?
Rebecca had a somewhat tough time in D-block. Not terrible, but not great. That being said: D-Block is and is not a good representation of her usual fights. It IS a good representation insofar as it involves an "out of bounds," and insofar as colosseum fights are likely often free for alls rather than one on ones. It is NOT a good representation insofar as the level of the fighters. This tournament is atypical - it is specifically a rouse to gather together some of the strongest people in this area of the world. Rebecca, while a prominent gladiator, is not used to fighting that tier of opponent (even if she can hold her own somewhat - I doubt she would have won D-Block, and in a real fight losing can be death).
Now take out the out of bounds, which is a staple of her fighting style - she dodges until she can position the enemy out of bounds. Without out of bounds, she loses her main offense. Not only is Diamante a cut above most of the competitors in this tournament, but he is most certainly stronger than the gladiators she is used to fighting.
Her fight against Diamante is a different kind of fight against a stronger opponent than she is used to. It is fair to conclude that she may simply be outclassed here, and that is why she may NEED protection.
That being said, I still don't like her. Her role is to be protected. That's lame.