Using Final SparkFinal Spark to hit enemies who are attempting to Dragon or Baron Nashor may allow you to scare them or even kill steal the Dragon kill or the Baron buff. Similarly, it can be used to finish off the monster when your team is attempting the kill.
It is advised for Lux not to get into a close-up fight, as her low movement speed, long cooldowns, and relative fragility will work against her. Stay behind your teammates and spam spells into fights from afar to stay out of harm's way.
Final SparkFinal Spark is great for clearing large amounts of creeps in a lane, and harassment in lategame due to its relatively short cooldown.
Feel free to use Final SparkFinal Spark just to shave off some of your opponents' HP, due to its cooldown and high damage output.
The damage of her ultimate Final SparkFinal Spark extends slightly past the targeting icon, but enemies will likely dodge it if they are at the very edge.
Consider using Final SparkFinal Spark at the beginning of a team fight to weaken the enemy team and give your team a big advantage. If the team fight lasts long enough, then you might be able to use it again to finish off foes running away.
Lux has prime lane control, making a Catalyst the Protector very useful to ensure dominance giving Lux enough health and mana. This can later be built into a Rod of Ages, granting a fair amount of Ability Power while giving Lux a good health pool should she be focused. The mana granted is also useful to stay in fights longer.
As Lux can stay relatively far away from a teamfight, AP should take priority over survivability. Rabadon's Deathcap is the most efficient way of doing this, making all her spells much more potent.
Since Lux's spell have long cooldowns, cooldown reduction is crucial to her utility/damage.
Since 拉克丝的Final SparkFinal Spark, has a relatively low cooldown and long range, it is best to stack up on AP and cooldown reduction to make it more potent and effective in weakening enemy champions before or during a team fight. Final SparkFinal Spark can also finish off enemy champions on low health that are retreating.
Building Archangel's Staff will grant Lux a fair amount of mana and ability power, allowing her to spam her abilities.
Since Lux generally stays behind her team in team fights and she can survive random chases due to her crowd control abilities, you can get Mejai's Soulstealer and manage to keep your stacks high, giving you a fairly high amount of AP and an edge over the opposing team.