@Domingo:
Depends on the "worse teammate" though. If they're willing to listen to other people and has awareness that they're weak, something can still be done. Kiting, or at worst, bail them out of enemy range until it's safe to join. Becca is the weak and obedient type, but also the one that thinks that she's the burden for entire party. In MMO term, being "useful" in party often equates to high DPS, save for the role of tank and healer. As in, if you do mediocre to low damage then you're useless, while there are still other ways of being useful.
Unless she changes her mindset, she will forever be a burden. If she can't however, then the spotlight belongs to Robin to do the supporting.
With the fluttering ground I don't think there is any way for them to get into a formation and it would be pointless anyway if one of them is weaker, since the one surrounded would step out to take out weaker one first anyway. It would be different question however if she had speciality as something that can be used for combo attacks, however she wields sword, same as his father, while her observation haki can only be used for protecting herself.
However as for change of her mindset, that's what I've been wondering to myself. I mean that Kyros have already maxed out his resolve, so pushing him over the edge wouldn't do much good at this rate, but I wonder about Rebecca's observation haki. She still isn't fully commited and might unlock a powerup if she actually got herself together indeed, but can observation haki alone compensate for all of lack of skill and power? Diamante flutters ground below, his sword, his mantle, everything around him works a s a weapon for him which gives him overwhelming advantages. Especially when his opponent is one legged. Perhabs it's another point for her since she can stilll balance with both legs.
I can see how Rebecca could at least show that he can fight decently if she commited to it. However the problem is that it's exactly what Diamante wants. He either tries to use her to make Kyros less careful and force him to make a risky move for protecting his daughter or simply intends to finish her off first so he can deal with already suicidal Kyros. I just don't see how in the current situation it would be considered as a wise choice for her to respond to that taunt.