Lux is an op champion support, if you main playing her you will not find any difficulty against any other suppor or counters.
Takeing ghost will give to her a bonus of movement speed to chase or run.
since lux is very op with her poke abillities you dont need to take exaust or ignite.

Flash is an essensial summoner spell to take it might save your life or use for a combos.

Flash : like every other champion does. It’s particularly important and your life depend more on It, its the most important summoner spell which is an indispensable, you cannot not take it, you can use it to save your life, Dodge a spell cast or to protect someone on receiving damage that could kill him or to get closer to shield/heal him also to engage a fight.
Heal : This spell can be used to save a life when low on health and under attack. It is commonly used to get an early first blood by healing oneself for more than one's opponent expects. It is also a skill used for "baiting", where you make your champion appear weaker than it actually is to lure your enemies out of position or to get them to tower dive you. A similar baiting tactic is also commonly used with Barrier.
Barrier : is great for absorbing the burst of enemy champions or turret enhanced shots. Whether used on a squishy mid-laner or a tanky top-laner, a well-timed Barrier will help escape from tough situations.
Barrier is damage mitigating summoner spells, barrier is not affected by Grievous Wounds, has a shorter cooldown, blocks more damage (against a single target), and can be used while you're still at full health (good against 100-to-0 burst).
Exhaust : is used against the player that deal huge damage most are adc to slow their attack speed, attack damage and movement speed.
Ignite as your second spell if you think you can win your lane or can duel a healer champion in your lane.
Ghost You can take Ghost to save your life once you got catched by a gank, flash ghost and you are safe, or to chase someone.

Pros

Can provide shields for team with her W
Can build Support or full AP to help team!
Goes well with most ADC's if played correct
Good sustain in lane
Powerful kit that is easy to learn
Super good poke
Ranged ult easy assists
Powerful early
Good CC
Huge range on all of her abilities
Deadly combos is you land a Correct Q

Cons

Cons
Long CDR to cast abilities
May miss skills shots and be affected
SQUISHY
Decently immobile
Positioning is important
Mana problems early
Skillshot abilities