Rumble is basically Garen level early damage with less of the fall off.
Yorick is basically a sustained rape train without brakes that's virtually immovable short of coordinated 3 man ganks of top, jungle, and mid.
And no, nashor's tooth is a horrible item in general. AP wick is basically a couple dorans rings, followed by a frozen heart, followed by general tanky items, usually a wits end first though, to keep him in the fight. The theory is heavy sustain and frequent % damage q's. Virtually immovable in lane, and almost impossible to outpoke. Either they have to give up their farm, and let you free farm, or get whittled down and, eventually, killed, usually via tower dive with your ult and a q to finish them off. If that's not enough, your ignite normally will finish them off anyways. You'll also leave the dive with over half your health left, usually around three quarters. Only exceptions to this are champions with range, so the ad carry with escape meta is pretty much a hard counter, followed by a few AP carries at top. Oddly, Viktor is one of them.
So yes, you don't really know how to play them :s