A witch is too vulnerable during their spells; they have to be quick and small during fights. Dark and grand spells that would instantly kill a target take from the witch too much energy and time, enough for the Slayer to take advantage to attack and win the fight. "That's the thing about magic: there's always consequences."

Also, it could be argued that Buffy didn't win the fight against Catherine and Willow earlier because she never wanted to kill these humans, only detain them. For example, Buffy was much more efficient against Kumiko Ishihara, a vampire witch: Buffy simply stabbed Kumiko and knocked the vampire down mid-flight, possibly killing her. And when Buffy fought Dark Willow again in an alternative future, she killed her former best friend by stabbing her with the Scythe.

I understand, so magic is not that advantageous, I saw that fight between Willow and Buffy and I imagined that there could be this possibility but there is not much chance, apparently the witches at this point are weak.