When Ahri’s Charm E is down, she has no real way of defending herself without blowing her Ultimate R. Look to fight her when her Charm E is not available.
Try and stand outside of the minion wave, but close to it. This will force her to choose between pushing with her Q or poking with it instead. Stand close to the wave so you can run inside of it to dodge her E.
Ahri is incredibly vulnerable when her R is down. Make sure you abuse the long cooldown early on.
As long as you build magic resistance and dodge her Charm E, she has no way of one-shotting you.
Ahri likes to roam after level 6 when she unlocks her Ultimate R. Ping her as soon as she leaves lane and try to follow if possible. If you can’t follow her, push the wave and try to draw her back to lane.
Ahri is incredibly vulnerable when her Ultimate R is down. Try to bait it out first, and then call for assistance from your Jungler to kill her while it’s unavailable. Make sure the wave is closer to your side of the map to make it easier for them to gank you.
Ahri is rather weak in the early game. Look to poke and fight her during this time to get a good start from the get-go.
Post-6, Ahri has a lot of chase potential thanks to her Ultimate R.
Do not overextend when you’re low as she can easily kill you and set you behind. Once Ahri has completed her first item, she will deal a lot of damage and be able to push the wave/ fight more frequently. Keep this in mind and respect her all-in damage.