If you wanna play Lux , you should know : Lux has got very usefull passive so , you should use it
, for example : if you catch someone in your light binding ( Q followed by E ) use Q E autoatack E(detonate) autoatack. Lux at level 3 can do about 300 dmg with this combo. Her ultimate , Q Ignite RE atack E(detonate) atack + and if it isnt kassadin or galio you got kill.

In teamfights you focus on dealing as much destroying damage as you can , so you will try to snipe , but how to snipe , easily :
use E , R , E(detonate) and if you hit more than 3 people you got to win this fight ... with serious team. but if you got a chance to bind somebody by your QERE(detonate). but sometimes let that Q for your safety because you don´t allways know who is behind you !

Lux is usuall pick for Ranked matches because of her big AoE damage and her supportive aspect which helps all the team to get kills. Personaly I think Lux like APC is the most skill depending hero in League of Legends . If you know her enough you are uncounterable , if you dont know you will fail , all is about practice , start at normal games end at ranked matches thats my way how I went to Ranked in season 3

+ Good AoE burst damage
able to instakill enemy carries for 3 spells even on level 18
supportive aspect
easy farming , if you have some patience at her autoatacks.
skill makes her hard to counter

- very squishy early game
item depending
skill needed§hard to play
bad agaist assasins
need good team which teamplays with you !

Early focus only on lasthitting and ganks , as you hit level 4 you can start harras because you have got protection , imobilize , slow , and your super passive , still keep focused on lasthitting .
At level 6 you can use QER just for force enemy to go back and get the creeps , or you can try to kill him with your Q ignite E R atack E atack . Chalice makes your mana not going out so fast when you spam E so now you can farm with E too ... very easy farming , in early game focus only on normal atacks

what to say

if you can catch 2 instead of 1 enemy , do so !
dont throw shield on enemy but on your team to prevent hard burst on them
Use your passive to make best damage what you can even without your ulti.. save it for sniping.
SNIPEEE SNIPE SNIPE and SNIPE !
Teamwork with your team and if your team works well with you you can win game so easily.