For masteries you don't want to access the Resolve tree. Since you're not regular Nasus, you don't really wanna build tanky in any way.

However its much better to go with the Ferocity tree, and the Cunning tree.

Ferocity will grant you alot of Damage, that is more for the late game. Were as Cunning will grant you alot of Utility and early game damage, and such.

This is why we take either Deathfire Touch , Stormraider's Surge , or Thunderlord's Decree . Which all give good bonuses for AP Nasus



- Stormraider's Surge -



Stormraider's Surge will for an example, give you the ability to follow up with a Siphoning Strike, or another Spirit Fire.
You can also use Stormraider's Surge together with Wither to completely eliminate the chance of escape, and compared to you -a super fast Stormraider's Surge empowered burst god- they are practically emobile.



- Thunderlord's Decree -



The other Possibility in the Cunning tree, is Thunderlord's Decree , which basically just increases your Spirit Fire burst, and makes early game poke a charm.



- Deathfire Touch -



Finally Deathfire Touch , this mastery is very good too, but is only really worth it in the late game. Because late game Deathfire Touch will deal 25% of Nasus's AP as DoT for 2 secs on his Spirit Fire or any other forms of damage, including his AA's.

Without counting in runes, masteries that increase AP, or anything simular, a Nasus's late game build will deal alot of damage. Example:

Frost Queen's Claim 50 AP + Luden's Tempest 100 AP + Liandry's Torment 80 AP + Void Staff 80 AP + Rabadon's Deathcap 120 AP = 430 AP

However because of Rabadon's Deathcap's passive, its actually 430 + 150 AP = 580 AP

That means a Regular AP Nasus's end game build will make Deathfire Touch deal 145 Damage over 2 Second, on no Cooldown.