MARKS

Nasus has scaling damage from Q,meaning that armor penetration is a must on him.Plus you take trinity,which helps you with sheen to deal clean damage on enemies.

SEALS

Scaling armor in my opinion is the best,you will take armor anyway from frozen heart.Late game you will be able to reach 210+ armor

GLYPHS

CDR early on Nasus is very important.His Q time can be reduced a lot if you start with 10% crd meaning you can boost early Q stacks faster,so you can duel better if you want to kill your enemy.

QUINTESSENCES

Lifesteal is important a lot mainly cause it fits with your passive,secondly cause it can give you more sustain in lane.

13/17 suits very well with Nasus,mainly cause you can deal damage and be able to defend yourself.Of course you need the 5% CDR as i said previously.Plus the health regen fits with your Spirit Visage and Warmog passives.

I max E because you can deal more damage with Doran's ring and also for the spam

If you want to make sure you can Q minions without leaving them with 3-4 hp you need to hit the tank minion 4 times and then use Q.On mage minions hit them 3 times and then Q.Cannon minions can be taken down when they are at 120 hp

In teamfights Nasus must focus adc/mage,mostly cause the deal the highest damage in the team and they are very squishy.You can reach them easily with your ghost/trinity passive and zone them hard.