Don't be overaggressive, don't sell your big items but buy a Reflex Block to block his stun and maybe some armor (not recommended) if absolutely necessary. The biggest mistake that most people make is that people forget that Koshka is an assassin, not a melee warrior/brute. You should get in, do a bunch of damage to their laner carry with your allies, and get out and wait for your abilities to cooldown again so you can go and finish off the laner(if he's not already dead). Also, always stick with your roam and remember that you should make sure that they get lots of scout traps and flares, because you should NEVER have to face check a bush because they didn't flare/trap it because there is a high probability that you will be ambushed and bursted down.

While stormcrown is awesome for camp clear, and gives you a small amount of unblockable damage, in my opinion Koshka doesn't really need it. Her cooldown is so small, her abilities coupled with the empowered hits afterwards, she doesn't need much help in clearing.

Stormcrown gives better sustain and some CD reduction but if you want to be more bursty and more like an assassin, more crystal items is the answer.

Around 2 for normal, low-reflex celeste because you can just burst her in an instant by PF and TD at once. If executed right shes a toast.
For smarter celeste that can check bush with Heliogenesis and being high reflexed can put Core Collapse right after you PF her the threat level increase to 5 or 6.