I'll just be the oddball who choses Robin...and this fight is why her ability is honestly a make it or break it ability....I mean Boa has shown no physical strength feats to over power an enemy just applications of her power.
Seems to me Robin would have the ability advantage. Considering she wouldn't be interested in Boa (between being a girl and the void century)would she even be affected by the Merro-Merro?!
And after that, Would she simply not just prevent Boa's hands from taking the heart shape?!
IDK....unless Boa can make anything that touches her turn to stone....I guess Robin should theoretically win...
So I'll just play the devil's advocate.