Ah, I think that ZSS rocks Rosa pretty hard. She may force ZSS to approach, but between zair, side b aimed at luma but not at Rosa (see: very safe distances), and the fact that many of our moves (bair, fair, nair, dash attack, down smash... heck even trading a flip kick) can one-hit-stun luma off the luma blast zones (the edges)... I just see ZSS having most of the cards here.
I'll note this here - ZSS can, if need be, go high risk on the luma kill and it's generally worth it. ZSS could flip kick, only hit luma, take a few shots from Rosa and it will still be worth the time gain. Why? Cause the resulting opportunity to punish favors ZSS way more. Also of note, Luma depends on Rosa's shield. If that reduces any, Luma gets shield poked first. ZSS' down smash is completely safe on shield 50% of the time and only vulnerable to 4-frame-or-faster responses the other 50% of the time. A short review of frame data shows that Rosalina does not a have a 4 frame out of shield response option. She needs a perfect shield to try and contest down smash. So, effectively, if Rosa fails to stuff the 20 frame startup of down smash, she can't effectively respond to the hit on shield. If the shield is small enough, Luma may take the brunt of that attack.
I recently learned that her dash attack (low to the ground, hits far away) has obnoxious amounts of end frames, and can be
shield grabbed by ZSS. This is not a normal response for ZSS. In fact, a light comb over of her frame data shows many moves having first actionable frames in the 40's and 50's with respect to end frames. As many know and have experienced, getting grabbed by ZSS is un fun. At certain low percents, you get the full b&b (dthrow, up air, up air, boost kick) and at extended percents you get the truncated b&b (dthrow, up air, jump, boost kick). And at even further percents, you get dthrow to double jump back air.
So... I believe that between having great tools for knocking luma out of the picture, her height allowing for rising aerials (not common for a character with such a large and quick short hop), and her moveset allowing for many kinds of
shield grabs (I cannot stress this enough!), I just don't see how this can ever work in Rosa's favor.
Granted, I see her strengths and how they lead to her being a top level character, they just don't add up to ZSS' in this case.
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Oh also! Since it's kinda my thing... I'll mention this: the
Hype Kick II can effectively stuff Rosa's recovery if timed right. The double benefit? Luma likely eats it even if you miss.