MATH TIME!

This is more of a build, not a guide, so have some MATH!
(Note: numbers are approximate)

Attack Damage from items:
485

Runes and Masteries:
485 + 34 = 519

Mastery Warlord - Increases bonus AD by 5%:
519 x 105% = 545

At level 18, Nasus has a base AD of about 113:
545 + 113 = 658

So Trinity Force's Spellblade's Damage would be:
658 x 200% = 1316

Plus you've got a 50% to deal CRITS!:
Crit Damage: 1316 + (658 x 250%) = 2961

Let's say Nasus' Q has 200 stacks!:
2961 + 200 = 3161

Q's base 110 Damage:
3161 + 110 = 3271

Don't forget your basic attack's normal damage with no abilities:
3271 + 658 = 3929

Mastery - Havoc: Increases damage dealt by 3%:
3929 x 103% = 4047

That's about a whuppin' 4000 DAMAGE

So you can go about and 1 hit their AD-Carry to halt the enemy team's damages (and don't forget you lifesteal 36% of that 4047 damage, which would be about 1457 health!), then 1 hit their AP-Mid so they'll stop Crowd-Controlling you which also gives you back all your health, then head over to the next possible squishy and 2 hit them, head to their tanks to 3 hit them, and go wreck their base!

#GGWP