This part makes no sense. How is defeating a greatly weakened opponent a testament to her swordsmanship or Haki?
Because it took down a low high tier. Taking down a low high tier(even weakened) in one shot is impressive
Robin was focusing on the children and wasn't trying to fight Monet.
Does it really matter? The most important part is that
1- The shot in question put Robin down in one shot, showing she has enough strength/skills to put her down with a hit
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2- She is faster than Robin by far. She went from fighting with Zoro:
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To casually blocking them(in their running away speed) with a shield:
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immobilizing one of them all that in the same process
And considering Robin lacks haki(to hurt her)+speed(monet being faster)+durability to withstand Monet's attacks, you do the math