Just revert this nerf, get rid of Haku's magic attack for the party, just don't kill the only thing that keeps Kanna viable for solo'ing...
You completely destroyed what you achieved with her last batch of buffs before this, Nexon.
EDIT: You could even keep the flat magic attack for the party if you see fit, but Kanna herself CANNOT play without the % magic attack on herself, unless you buff a bunch of her skills. Be reasonable, please.