Hi my name is Cpt Kirk and I am the Captain of the SS Enterprise. I am also a Low Diamond high Plat ADC Player who has been playing LoL since Season 3. With this Marksman Update ALOT of things have changed and I hope this Lucian guide helps you with what I believe is the most powerful adc in the meta. :)
Please comment on item sets you think are good and voice your opinion. Also dont forget to give me a thumbs up :)

PROS

Level 2 Power Spike
Essence Reaver gives CDR and attack speed
Marksmen OP
Highly mobile
The Culling does a lot of damage
Very bursty and has low CD

CONS

Low Range
Needs help early game
Weak before Essence Reaver
Mana hungry early

Runes haven't changed in the new Marksman update so standard runes should remain the same. I take Flat AD quints because with the new item updates it is easier to obtain Attack Speed.

HURRAAAAY for new masteries. Here is a change. With thunderlord being on roids i take %dmg and TLD and still use attack speed and lifesteal in offense.

http://www.mobafire.com/league-of-legends/mastery-tree-planner#&tree1=5-0-0-1-5-0-0-1-0-0-0-0-0&tree2=5-0-0-1-0-5-0-0-1-5-0-0-1-0&tree3=0-0-0-0-0-0-0-0-0-0-0-0-0&v=2

Flash: This is the be all and end all of summoner spells. You need to take Flash it is your escape tool and can be used offensively to secure kills.

Heal : I prefer heal over anything else because with the ignite nerf your allies get the full value of your heal. Also it gives some movespeed to run away :3

Barrier : If you do not have a support with heal and are planning to skirmish a lot take this.

Teleport : Take this if you don't have a support or are planning on farming ALOT.

Cleanse : Lucian is a very mobile adc and has a dash that can help him escape from alot of cc. If you are up against a lot of cc such as Leona or Sejuani then take this instead of Heal

Lightslinger
This Passive is amazing and is what makes Lucian level 2 power spike so deadly. Any skill you cast is followed with a double auto attack. The auto attacks help reduce the CD in Relentless Pursuit by 1 second if AA hits creeps or 2 seconds if hit on champions. This passive increases your damage significantly and helps you become more mobile with Relentless Pursuit

Piercing Light
This is your main damage skill outside of your ultimate. Use this to farm caster minions and poke the enemies at the same time. Try to use Q when you see the enemy line up with cs.

Relentless Pursuit
This is your escape tool and your gap closer. If enemy is low engage with this, make sure you use your passive and combo with Piercing Light and Ardent Blaze. This skill can be used to go over certain walls.

Ardent Blaze
This skill is used for getting your passive off and gaining that short burst of move speed. Put 1 point in it and max this last because it doesn't do any damage and helps reset your Relentless Pursuit

The Culling
Alot of people think that taking away the attack speed portion from this skill was a nerf but trust me this is a buff. At level 6 you can do over 1000 dmg. Use Relentless Pursuit to make sure every last shot hits.

Early Game

In the early game Lucian has moderate damage output. Besides his "level 2 power spike" Lucian doesn't have a lot of damage until he hits level 6. In this early phase focus on farming and hitting enemies using your Piercing Light. If you have a support [Thresh] or [blitzcrank] and they happen to land a hook use your Relentless Pursuit and dash sideways, use your passive Lightslinger, then follow up with your Piercing Light followed up with your passive again. This will guarantee that you win 90% of trades.

Mid Game

During the mid game you are a beast. Essence Reaver has been completed and you can basically 1v1 almost anyone in the game. During team fights stay behind your team and follow up your teams cc with The Culling and melt the squishies and tanks.

Late Game

Lucian late game is insanely strong but you must be very wise in how you use your abilites. Use your ultimate to melt tanks and squishies from afar and only use you Relentless Pursuit to kite, dodge CC, or secure a FREE OPEN KILL. DO NOT use your E offensively, it is your biggest defensive tool and will make sure you are safe and help you get all your damage out.

Farming with Lucian is a little tricky, but if played enough, becomes a very powerful skill that can be used for many purposes. The first rule of any ADC is to stand behind minions when farming. This will protect you from Thresh and Blitzcrank hooks and prevent you from losing mana by using Relentless Pursuit. With this being said sometimes it is very risky to farm caster creeps. Fortunately, Lucian Piercing Light makes this a problem of the past. Lining up Piercing Light takes a lot of skill and practice and sometimes you may miss. Take the time in normals to understand the geometry of this skill. Also Q can be used to harrass enemies that hide behind minoions. Use this to your advantage and maximize your damage output.

Also your passive Lightslinger can help with farming aswell. If two or three creeps are low and you do not have enough attack speed simply Piercing Light one of them and use your passive to kill the other 2.

Finally if there are 5+ creeps amassed in a wave simply use your Q to line them up and take them all down at once. Piercing Light does not lose damage the more enemies it hits so you can clear large waves fast.