Tristana Can't Focus Targets

I tried going hypercarry tristana once because i got gunslingers and kept having her show up on my roster, so thought eh why not. The problem with that is Tristana literally can't focus a target worth a damn. Almost every other hit she is hitting a different target, making her damage spread out, so while enemy team is focusing down my front line Tristana gets barely anything done because she's not capable of focusing a single target til it dies, which she needs to be able to do. I mean focusing down a tank is much preferred then just hitting a different target every shot.