Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Farm with spirit fire as much as possible, also remember to last hit with your siphoning strike, then the damage will increase by it self, use it also to hit turrets. I max wither at lvl 13 to sure the enemy champs doesn't flee or keep following you. Beside i get high movement speed, it only makes it that harder to flee from Nasus. Never be afraid of tanking, cause you can, but also hit really hard with siphoning strike activated. Chalice i buy due magic resist and the mana regen. You use your mana quickly but again, you end op with an okay mana pool, due trinity force and banshee's veil. Also take the golem buff, with it i once had a 154 mana regen pr 5 sec ;O summoner spells it not a must, ghost instead of teleport works fine, i use the teleport to surprise push. Exhaust also works fine, just give you another slow. Seen that flash also goes good with Nasus, so find out of it yourself which summoner spells that works for you, Have fun :)