well I've played around with ap varus for awhile so here's my experience of items that seem to work. W does scale with your ap and fairly well because it does a %hp for damage. If my memory severs me correctly 400 ap will hit for 50% of the enemy's health for each 3 stack hit. So 3 stacks of corruption use E then 3 stacks and then snipe with Q and dead. Period. But it's hard to get that much power until late game and varus ap is really weak early. And as for items I went with BotRK, Hurricane to spreed your E, NT for speed and on hit effects, and RD is a must. everything else is pretty much up to you. now that being said NT and RD will not get you too 400 ap, but somewhere around 200+ ap i think. And AP varus dosent have a whole lot of sustain but HUGE burst. so try this 3 stacks, ult, 3 stacks, E, then Q snipe if needed.