Irelia: she's obnoxious to deal with and if she's skilled then all you can do is sit and farm under turret and hope she doesn't either dive you or freeze your lane. She'll most likely kill you if you walk up to CS if she has ult and the wave is frozen.
You can take exhaust, but the lane is over for you either way, if she's skilled.

Kassadin: He outscales you if you don't punish him early game. It's a free lane pre lvl 6 but after that it's a skill matchup

Yone: skill matchup.
Stay away when he has wind around himself. If he Qs forward, Q him. He's stuck in that animation so hitting him should be easy. Then you can get a good trade off.
As per usual, E when he walks up to CS but keep your Q in case he engages.

Akali: fear this is yet another disgusting champ with a thousand different dashes.
You win super hard early game, until lvl 6.
Have a minion between you so she can't E you. If she hits her E. Q her when her shuriken spins over you. She's stuck in that animation so you'll automatically hit her.

Xerath: he's annoying to deal with because he outranges you, but he doesn't have much kill pressure

Zed: q behind you after his ult and trade whenever his shadow thingy is down.

I will be adding more matchups later and moving everything around to make it alphabetical order and a more general guide based on what type of champion it is

Autoattack the first wave once or twice so you can last hit while getting a slow push, which you will naturally get because it's a ranged vs melee matchup. Poke them when it's safe to do so and E when they walk up to last hit.

You will usually want to contest for level 2 first. Push the wave with your auto attacks NOT YOUR ABILITIES. E whenever they walk up to last hit. If you don't get the push, back off until they've used their abilities or you get lvl 2.

Cosmic drive:
Can be good if you need to space glide

Imperial mandate:
Can be good if you are behind and go more utility-focused. I rarely build it on mid.

Oblivion orb:
Good if they have 3+ champs that heal a lot.

Morellonomicon:
This item is absolute garbage. Do not build it unless it's your last item. Just buy Oblivion Orb and move on, it's a waste of gold.

Staff of Flowing Water:
It's a good item if you're playing from behind and go for a more utility-focused build. Works well with Lux's AoE shield. The movement speed buff is always good and the AP is at least always good for Lux and other champs with AP scaling.

Shurelya's Battlesong:
Never have I ever built this item on mid, but I think it could be good.


Zhonya's Hourglass:
It's good vs dive comps and heavy AD.

Mejai's Soulstealer:
I build this almost every game. Upgrade it from Dark Seal if you hit 10 stacks and are ahead. It can be risky, but Lux has a high range and can easily stack it.

Seraph's Embrace:
I do not build this very often. It's mostly a filler item. I only really build it when I go enchanter or I build Tear into an item that doesn't give mana.

Stormsurge:
I think this is a bit of a bait item. Try building it and at the end of your game, look how much it has done. That being said, I like to do a little bit of trolling and so should you.

Shadowflame:
I fear this is another bait item. When are you ever going to be getting that 30% crit, huh? You could do something like Q/E1>E1/Q>R>E2 to make your E benefit from it, but why would you do that when you could just build Horizon Focus?

Banshee's Veil:
This is a good item vs AP and **** like Blitz Q, Ashe's ulti, TF's yellow card etc. That being said, it was mostly built with Crown of a Scattered Queen back in the good ol' days because they had good synergy.

Liandry's Torment:
Once again, in this cancer szn this item no longer gives mana and feels bad to build unless you're up against a **** ton of tanks.

Malignance:
I fear this is another bait item (kind of). Buy it and look how much damage it did throughout the game. That being said, it's still a good item because of the bonus ability haste to your ultimate. With this item you can shove waves like crazy and your ult will have a 20s CD in the late game.

Luden's Companion:
It's a decent item, I hate it.

Lux's damaging spells (E, Q and R) charge the target with energy for a few seconds. Lux's next attack ignites the energy, dealing bonus magic damage (depending on Lux's level) to the target.