'ave some sprites

[Vipershark]

That doesn't really have much power behind it. She barely moves her body (and as you said, her feet) so there's minimal weight being put on the sword.

I realize it's a quick slash, but... it just looks wimpy.

Edit: Rather than slashing straight down, she should be slashing diagonally down to the right (or clockwise) since she's left handed. She should twist so that her torso faces more toward the camera until the end of the slash, at which point she turns back and pulls the sword back to her shoulder.