D Cane needs stacks. But this is ARAM, so put down the cane, and lube up your E key.

AP Nasus works exceptionally in ARAM thanks to his strong zone control and high damage. Unlike on Summoner's Rift, enemies will find it exceptionally difficult to dodge your E as it takes up approximately 1/3 of the lane width. With this build, it will be available every 7.2 seconds whilst remaining on the field for 5 seconds...for a total of 2.2 seconds downtime. This adds up to an insane amount of damage over time.

He also provides strong peel by essentially snaring anyone diving him with Wither. Lastly,

A masterful AP Nasus weaves in and out of the front-lines like a dance.
When your E is available, waltz your way into a bush and wait for an opportunity to poke when someone is farming. When you have used it, retreat and prepare to peel for your backline with W. Simple.

This core strategy is almost too obvious to explain, but is the only secret to success with this guide. Most of your enemies will fail to adapt to it.

Always focus on squishy targets - this is usually easy as they like to walk forward to farm. Get them low and collect those free Dark Harvest stacks. If you are feeling particularly adventerous, go ahead and wither them and let your team engage.

To accomplish this, take control of the bushes and make sure to group up with and stand behind your tank.

If you have built correctly, your E should be doing moderate-high amounts of damage to squishies but abysmally to tanks.

Teamfights
Always use your E first to divide enemy tanks from their carries. As everyone clusters, try to aim it in front of the carries and behind the tank, prioritising hitting the carries. After your E is on cooldown, you should assume a backline role in teamfights, using your ultimate to shred tanks for your carriers whilst peeling with Wither.