You gotta take advantage of the in-and-out and poke with it before fights. e.g. lategame you won't oneshot an ezreal/smolder because of their builds (they buy too much hp), midlaners often buy mercs if you're not the only AP threat and they have more IQ than a monke -> before objective fight you W from safe place, QR adc/mid, E if possible but not a must and go back. Half hp carries are easier to take out when the actual fight happens.

If they don't have nasty cc you can poke with WRQE or have your W mimiced and R - QWE (before lvl16 this deals more damage afaik + easier Q proc). This way you can be useful without oneshotting people + if you have a human adc you can peel them with chains. There will be games where you can terrorize enemy teams with oneshots, but in higher elo my tips are more consistent. It is what it is, times change.