There are a few easy ways to nerf Tristana and a bunch more without fully harming them.

You can nerf stattik shiv magic damage or its power with Rapid fire cannon

And you can turn Tristanas ult into physical damage that way shes not blasting you with magic when you built armor against her if shes fed.

Not every nerf needs to break a champs ankles, i dont know what you guys have planned but im just saying these should be looked at first.