Runes: The big ones here are Magic Penetration Marks, and flat Ability Power Quints. Some people take defense for Seals and Glyphs, some people take some cooldown reduction for Glyphs, and some people go full greed, and get scaling ability power. That choice is up to you.
Hope this helps at least a little!
Note: For the highest damage possible, you'll have to look into items like Seraph's Embrace, but I don't recommend this in a practical sense, as the build path and ramp-up on it are terrible on Lux.
If you're getting Abyssal Mask for the MR you'd do it earlier in the build. It's not a great item on Lux because her spells heavily outrange the aura. I'd only build it if the alternative was dying repeatedly.
There are a few things to consider on
Runes: The big ones here are Magic Penetration Marks, and flat Ability Power Quints. Some people take defense for Seals and Glyphs, some people take some cooldown reduction for Glyphs, and some people go full greed, and get scaling ability power. That choice is up to you.
Masteries:
For most burst mages, you'll want 12 ferocity and 18 cunning, taking:
and
For items, you'll want to open with a
If you need more mana sustain,
Otherwise, you can build straight to
From here,
Hope this helps at least a little!
Note: For the highest damage possible, you'll have to look into items like