Both level 18, Yi with Last Whisper and BotRK vs Rammy with Thornmail and full tank build/ runes, Yi wins the 1v1 encounter by dealing upwards of 1000 true dmg. Try it in custom if you don't believe me. Roit, pls nerf his ability to kill tanks :(
Why should Rammus be able to 1v1 Yi? Rammus is supposed to be a utility tank. Theoretically, he's heavily reliant on his team to actually deal the damage, while he's soaking fire and disrupting targets. Yi, meanwhile, is designed to be a 1v1 assassin who can quickly pick out of position, isolated targets with his ult.
Do you think that Sejuani should be able to duel Jax? Do you think Naut should be able to duel Vayne? Why do you think Rammus should be able to duel Yi? Just because he's designed to punish auto attackers doesn't mean he should be able to win any duel against any auto attack champ in the game, just like Galio doesn't necessarily win every duel against every magic damage champ in the game. They're both utility tanks who aren't really meant to be 1v1ing people anyways.
Now tell me what happens in a 2v2? Because that 1v1 is going to take long enough that their entire team can get back from a coffee break and join in on the fight if they want.
Tanks are supposed to become less effective as the game wears on, I see no problem with a hard carry defeating a tank at level 18.
Why should Rammus be able to 1v1 anything?
You do LITERALLY 90 true damage at max rank for a short duration. Other pen comes from items so I don't see why Yi is in the title.
Lets just say that i'm not really okay with the power of Master Yi as a champ. Rammus doesn't need buffs, don't get me wrong. If anything, i think Rito should revert Rammus' last buff.
Maybe i'm just sore about true damage attached to auto attacks.
The fact here is that Rammus is purely tailored to counter what somebody like Yi does, and he cannot, because Yi is too strong.
I wonder if AD Irelia wins vs Rammus? Doubt it.
Better nerf 
...
I call BS.
I have played hundreds of ranked games with rammus, and at full build a yi has never came close to killing me.
What is 1000 true damage when Rammus has 4k + health.
Just rough numbers. A rammus at 18 will deal around 110 magic damage per auto attack to that yi. His ult will deal about 200 damage per second to Yi. To make it simple lets say he lifesteals the same amount as thornmail deals back (and that is being generous).
Lets say yi is attacking 2 times per second. At 6 seconds that Yi would have taken roughly 2500 damage just from Rammus W+ R. And Yi is not going to cut through rammus that fast.
Now , if Yi builds 100% tank killer, and Rammus itemizes a couple items for MR... then maybe, maybe, he has a chance. But when rammus stacks armor and thornmail, no AA'er can kill him 1v1 without superior kiting/ outplaying.
Rammus dies before highlander ends and wuju style ends.
Rammus should theoretically beat all AD characters. In practical situations he takes it up the rear with no chance of victory.
