Lux really stands out as a long range champion. She is best picked against shorter ranged opponents, especially those who use melee ranged normal attacks. She is also strongest against champions who lack mobility as she is far more likely to land her skill-shots. She can be played effectively as a Support or an Ability Power Caster. Her burst damage is not top-tier in the sense that its very rare you'll be able to immediately combo higher-survivability players unless you're doing very well. Instead she can consistently harass her opponents and usually do-so from a safe distance. As Lux lowering you're opponents health with harassment is often necessary before comboing them for a kill. As always CDR works best on champions with short cool-downs as a means you'll realistically get in more spells during team fights, all of Lux's spells including her ultimate fall into this category so be sure to consider CDR.