Since you are leveling up, and may not have keystone masteries yet, they will be harder to kill.
That being said, Executioners Calling is an 800g Attack Damage item that put's grievous wounds on the target, causing them to heal for 40% less. This shuts down life steal and Yi's W, and it shuts down Warwicks passive and Q.
Morellonomicon does the same thing, but only when magic damage is applied to a target under 40% HP, as opposed to Executioners which applies as soon as you deal any physical damage.
%hp is great against warwick, and to a lesser degree yi, but Yi is a late game hyper carry, so he is meant to do a LOT of damage in a 1v1. The best way to stop Yi is to crowd control him while your teammates burst him down, or play someone with a lot of up front damage like Syndra, Fizz, Talon etc. My personal Yi counter when I jungle is Xin Zhao, as he is a lot like Yi, but tankier with more Crowd control. Xin should win any 1v1 at any point of the game vs a Yi, as long as Xin went jungle and got the red smite item.
Warwick is a tank with a lot of sustain, so sustained DPS from someone, again like Xin Zhao, Kayle, any AD carry, or champions with sustained magic damage like Cassieopeia, Rumble, Swain, Fiddlesticks should be able to beat him 1v1, given that you can kite well, or can build some defense (which Swain and rumble naturally do for their builds). Warwicks healing also doesn't kick into gear very much until he is already low HP. Save your burst if you are on an assassin/ burst mage until he is around 40-50% health and then try to blow him up.