This build is, as the title says, designed to be an AD-focused build with Nassus for the purpose of being able to take out the enemy's carry(s). Combine Nassus' natural taniness with the slow of his Wither and the massive damage output of a farmed Q and you, too, will be able to blow away enemy champs within seconds.

Early game, your focus should be on farming Siphon Strike as much as possible while not pushing your creep wave. Use your mana pot to keep your Q available, and your health pot and Wither to keep enemy champs from being able to cut you off before you can retreat. You should be able to stay in-lane until at least level 6. If you have to return to base, or decide to to buy, remember to use your teleport to get back in lane as quickly as possible.

Mid and late game, you should continue farming your Q as much possible while pushing enemy lanes. Map awareness is key - one champ can easily be escaped with wither, two are avoided with ghost, but more than that and your best bet for escape is your teleport. Swapping pushing top for bottom right before an attempted gank and vice-versa is a good way of continuing to apply off-balance pressure to the enemy while continuing to farm your Q and push.

AD Nassus is an effective sidepusher and damage dealer during teamfights due to his ability to take and dish out damage while focusing almost entirely on damage/crit items. His wither is excellent for reining in fleeing champs, but the real meat of the build is his Siphon Strike. Combining Nassus' Wither on the enemy carry and his well-farmed Siphon Strike results in a QQ kill on all buy the most armored tanks.

AD Nassus' presence in teamfights are a must. As anti-carry, it's your job to target the enemy's most dangerous, escapable carry and crush them with a combination of Wither, Spirit Fire, and multiple Siphon Strikes. Armor runes, Aegis, and Nassus' naturally tanky nature enables you to wade into the fight (just don't try initiating). His Fury of the Sands is an excellent way of getting a quick HP boost, and depending on the skill of the opposing team, can be useful in the order as an additional helping of damage to ensure your survival when wading in to that carry.