Mei can increase and decrease the heat of her boil release. Theoretically, she could turn it up to a degree making it an instant kill. Therefore, Kakashi wouldn't have time to even react.
Theoretically kakashi is much more inteligen then mei virtually Kakashi is 10 times better with his kamui and all Jutsu
Chidori
Demonic Illusion: Hell Viewing Technique
Earth Release: Double Suicide Decapitation Technique
Earth Release: Earth-Style Wall
Earth Release: Hiding Like a Mole Technique
Eight Gates
Evil Sealing Method
Eye Mind Reading (Anime only)
Fire Release: Great Fireball Technique (Anime only)
Front Lotus (Anime only)
Genjutsu: Sharingan
Hiding in Mist Technique (Anime only)
Kamui
Lightning Beast Running Technique
Lightning Cutter
Lightning Release Shadow Clone
Lightning Transmission (Manga only)
Multiple Shadow Clone Technique
One Thousand Years of Death
Rasengan
Shadow Clone Technique
Shadow of the Dancing Leaf (Anime only)
Silent Killing
Strong Fist (Anime only)
Summoning Technique (Ninken)
Summoning: Earth Release: Tracking Fang Technique
Water Clone Technique
Water Release: Great Waterfall Technique
Water Release: Water Dragon Bullet Technique
Water Release: Water Encampment Wall
Water Release: Water Shark Bullet Technique
how can she stand that? first kakashi uses Hiding in Mist Technique then Chidori or kamui.