Warwick is an AMAZING champion, and in this guide, I'll teach you how to 1 shot most people with warwick's ultimate.

For runes, I take COMPLETE armor penetration. That's all I take.

For items, I first get a cloth armor and 5 potions for jungling, and once I hit around level 9 , I should have a Madred's Blood Razer, at this point, your ult is STILL pretty weak, so go ahead and grab a malady. After you get a malady, take a black cleaver. If the game still progresses and they haven't lost. Grab a trinity force.
Here is your damage.
When you use your ultimate, let's say someone has 2000 hp, the standard amount, your ultimate instantly suspends, and madred's kicks in, cutting off 20% of their hp, he has 1600. He now takes 800 from your Trinity Force, and his MR is reduced by 40 by malady, AND he takes 100 damage. His hp is now 700, now your ultimate's base damage PLUS the bonus damage should take him out. Because his MR is dropped so low , and the target's armor is really low as well, he should be short work by your team if he survives.