Hello and welcome to my first Guide. I hope I can help you a bit to improve your gameplay against specific champs with Lux.
I play her a lot and I am confident about my opinion. Better late then never ;)

If you have any question that I didn't answer in my Pro and Cons chapter, feel free to write a message to me.

Thanks a lot and see you soon on the rift.
AyUmUNiquE

Pro:
+ Runes: Summon Aery --> most of the people forget that Lux has a shield. With Aery you get a bonus shield and that helps you a lot against poke champions. You can stay on late even when you don't have enough HP. You have a higher chance to stay alive when you get tower dived. Also Aery is the perfect thing for sniping because mostly they stay alive with really low HP after using your Ultimate, so that will mostly not happen anymore.
+ The Ultimate Hat --> everytime you use your Ultimate your Cooldown reduces. When you kill someone (which you often do when you snipe someone with your Ultimate) your cooldown gets down to 12sec at Lvl 18.
+ Absolute Focus --> If you fight against someone and you get a gank from your Jungler and your enemy flashes out, your will mostly stay above 70% health, so this rune will give you some bonus dmg for your snipe.
+ perfect timing --> best thing against a Zed. You don't need to be scared about him reaching Lvl 6 before you. Just don't forget that you have your Stopwatch!
+ future's market --> even when you get zoned out or get behind in the early, you can still get your core items and won't stay behind that long.

+ Items: Lost Chapter --> helps you to not be OOM (out of mana)while poking. make sure to get this as early as possible but push your lane out before going back.
+ Sorcerer's Shoes --> you need them because if your enemy is smart, he/she will buy magic resist but if not, you get some easier snipes even on tanks in the early.
+ Any Magic Penetration Item --> easier snipes as well

+ Gameplay: press your Ultimate early in the fight on the enemy carrys so you get it again at the end of the teamfight to snipe someone who got away low. Poke your enemy laner with AA (auto attacks) and wait for Aery to come back, then AA again --> easy poke with more early dmg and less cooldown then the comet.


Cons:
+ Runes: don't be too over aggressive because you have to wait until you get your stopwatch.
+ Gameplay: stay behind your team, don't splitpush alone, make sure to not get catched in the jungle, get your combo on point