Ap varus is really fun but you have to know how to play him. the crux of AP Varus is his w. The whole reason this build is good is because you do max heath percentage damage. Eventually with enough ap you can one shot someone.

In lane you want to hit some poke on the enemy but as long as you farm well and proc first strike when its up you'll be fine. The way varus' w works is that you want to always use your q to proc his w blighted stacks. This is because when you charge your q and hit someone with it you can get up to 50% extra damage based on charge time when you hit someone with stacks of blighted on them. This is the reason you should never proc your stack of blighted with your e first. the whole point of your e with this build is to slow them down so you can kite and thhen blow them up. Whenever you hit your R it applies stacks OVER TIME meaning its better to charge your q up and slightly risk your opponent becoming unrooted. Don't use your R unless your sure you need to early. your abilities cooldowns are reduced when you pop your blighted stacks, a lot opf people dont knopw this so use it to your advantage when your fighting someone. when you first get your ult you want to save it for a guaranteed kill with a gank or for a tool to use against hostile ganks. later in the game this become less important because your ult gets to be on a pretty low cooldown. when you find yourself in a tough spot against a melee champion pop your ghost and kite them out, there's not much they can do against it.