A couple of months after that Mikoto would fight a true Multi Skill and she would still be holding back.
We know her opponent only has the upper hand because Mikoto isn't trying to hurt her for real.
I suppose because the story can't have Mikoto to be too ruthless, her powers also seem to expand to allow her to use it in so many ways. Electricity is pretty destructive, in the end, so it's a difficult match for someone who doesn't want to hurt anybody.
Sha isn't even worth analysing. She's as ridiculous as any character who can endlessly steal other characters' skills and immediately use them with extreme finesse. It's like the years of physical/mental practice and innovation the original skill owners went through is worth absolutely nothing. The cheapest of cheap writing.