You might think why rod of ages and not luden's echo
1. rod of ages when max stacks has more ap
2. rod of ages costs less (thats' the main thing when you will spend less gold on first item you will be able to get your second item quicker)
3. rod of ages doesn't have cdr but hextech glp-800 will give you the cdr you need to get to max cdr .
4. rod of ages has less mana but after you build hextech glp-800 you should have enough mana to use a lot of skills.

On start you buy doran's ring or spellthief edge.
On first back you should want to buy sapphire crystal and boots. (650gold in total)
On second back you should want to buy blasting wand (it will help you to deal much more dmg early game )
Then continue building rod of ages after rod of ages you should want to buy rabadon's deathcap it gives you much more ap and helps you take out enemies easier if team fights start when you on your 1st ,2nd item
I usualy max out my boots at this point i do it this late bcs i usualy don't need speed or penetration and getting my first 2items is mostly more important
Then buy the lost chapter (it will give you cdr which you might need at that point)
Then continue buying hextech glp-800 ( it should put you on max cdr with cdr from runes and give you mana to be able to spam your skill's more often and ap is never a bad idea on lux)
After that you can buy what you need the most either spellbinder for more ap or other items.

If champ can outrange you or outdamage you it will be a threat to you but if you outrange someone you have a big advantage . Easy lanes should be when you against tanky champ which deals no damage or champ with very low range when you just need to be at safe distance and with more range deal a lot of damage

If a champ can stun or deal a lot of damage you will be a good combo because it will make it easier to take out enemies either in bot or during ganks in mid.