Deathfire cannot be a substitute for magic penetration. Many tanks include magic resistance in their runes and come late game they'll likely have some form of magic resistance. Other carries may opt for banshee's veil as well. Deathfire is still magic damage though. It's not true damage so it can and will be reduced.

As for the defense...get something like hourglass. it'll give you 50 armor and 100 AP plus an unique active. Having that is going to give you 3 areas you could benefit from and greatly increase your survivability during teamfights.

I don't mean to imply there is only one way to build Lux, but I just think you have room for lots of improvement here. Especially the whole Soulstealer / Lich bane combo with out defense. That probably bugs me the most because any competent team will take advantage of how squishy lux is

Vaestryn, although health might not save the say, Magic Resistance and Armour can :D

unfortunantely I have to agree, this guide isn't very good, no defence etc.
just the fact that u started with amplifying tome is kind of stupid. Doran's Ring is a much better choice.

First of all, thank you for the criticism, I welcome it.
Deathfire more often than not solves the issue with late game lack of magic pen boots.
As for the defensive issue I find that wasting my time on beef with this character isn't a good idea: No matter how much health you end up giving Lux when you are in a situation where you need to run away I doubt your health will save you if your dealing with someone who knows what they're doing. Personally I also don't prefer the movement speed increase because if your doing your job right, you'll be able to root and slow your chaser.
On your point with her being a support, a good support can leave the lane when fights are done and return before the next begins.
The potency runes are there for early game heavy AP, giving yourself an advantage over others is a tool to prime your early game and get ahead.
Why on earth however do you think I'm trying to insult Lux? This merely the way I play her, the way that works for me unfathomably well because it compliments my play-style; I'm merely here ranting about it and hoping others find it useful.

Congratulations, you’ve done just about everything I hate in how to build lux.

Potency seals are very low-ranked as they give very little benefit. Clarity runes are generally preferred for casters for good reason. You receive about 5 AP from your seals to start whereas Clarity runes will give you valuable Mp5/lvl. I wouldn’t recommend putting more MrP Glyphs as again, I think you’d be better off going for CDR or my personal choice of AP/lvl.

Movement speed in utility is more important than mana base (see archangels).

CDR boots are bad come late game when you need the valuable MrP. Mejai’s soulstealer should be based on a game to game basis. You have no Clarity runes or Clarity summoner spell which just makes me think that your mana regen is goin to be horrid once you tap out of your max mana. Lux is support, she shouldn’t have to constantly back. Lich Bane putts you in a close range of your enemies and just asks for them to target you. Archangels is kinda meh. Lux is not Ryze, she does have mana problems and giving her a ton more mana does not solve this. Additionally her cooldowns are extremely long unlike Ryze and other casters so it takes a lon time to max out Archangels passive.

Furthmore, the 3% of your max mana being converted to AP....the 5% increase you get from your masteries that you chose....Say you have 3000 mana. 5% would increase your mana by 150. 3% of 150 is 4.5 AP. You’re only getting an extra 4.5 AP and 150 mana through that mastery where as movement speed can always be used. Increased survivibility is a must, which brings me to my next point.

You have no defensive items. Need I say more? Well, just to dig your hole digger. If you’re bringing a Mejai’s and Lich Bane to the battlefield, you WILL be focused and dead with no defense.

Sorry, but was this theorycrafted or have you even tries this? Or are you just insulting Lux by saying this is capable of playing her to her full potential? I honestly don’t know. This build seems very heavy.

Downvote.

Why did you put Lick/Archangel before Rabadon's?

Curious