Why are you nerfing Tristana's Damage? That's not her problem.
Before her problem was she didn't do enough damage at various points in the game to be a worthwhile pick, it made her power curve very strange. Like a "U" shape but the high points in said shape were not high enough.
Now that her damage feels appropriate, your issue is that she has a long range resetable leap. Tristana cannot be allowed to have this leap without major changes to the leap itself.
Again, her damage isn't the problem. It's the leap. You need to target the leap with some changes to make it so she doesn't snowball so hard. That leap is the only thing that's making her godlike. Target that. Stop skimping around it. You know it, and the rest of us know it.