On lane your playstyle is somewhat similar to an oriana, you have very high range, good poke dmg, but no disengage. Play safe, grab cs. When you hit level 6 you become a threat cause your poke is deadly. Don't engage in combat with enemies, keep distance and use your ult after hitting e, with slow your ult is a bit harder to dodge.
When using your ultimate you have to keep in mind that it's easily dodgable, so you shouldnt just try to use it on top of enemies but predict their movement. You can miss all you want as long as you dont start spaming your ult. The first cast costs only 40 mana but maximally charged its 400. The damage doesnt nearly compensate for bonus mana spent.
In late game, your ult gets stupid range (if you know numbers its 1800). You can poke enemies without even seeing them. You reach the reset range of dragon from outside the pit without breaking a sweat. NEVER get close to enemies if you get hit by any cc youre dead. Use your q and e on front line, and ult to disturb backline.
Full build your one combo does unexpected damage (q,e,r-2700 dmg to 100 magic resist enemy) so you can kill assasins coming for you or assasinate an adc in a blink of an eye.

Items in the build are optimalized to do maximum damage (except zhonya, but without it playing ap kog is impossible). Youre not supposed to get in range for being hit so youve got no hp to survive it.

You need archangel's staff because youre very lacking in mana. I dont build much ability haste because your ult has its 8 second discharge time and you cant just spam it.

Take ghost and phase rush to not let your enemies get to you.