How do I scratch a full tank Udyr as Sivir?
His Build - 
Mine -

Can we nerf this broken item?
Lol but seriously here was what he build

I felt like I was firing toothpicks at him. Toothpicks that he'd shield.
His Build - 
Mine -

Can we nerf this broken item?
Lol but seriously here was what he build

I felt like I was firing toothpicks at him. Toothpicks that he'd shield.
I would probably switch out cleaver for a bloodthirster. With a build like his, udyr would shrek you just because you would be damaging yourself more than him with that thornmail of his.
You didn't have much lifesteal, so he could probably kill you with base damages and Thornmail. I'd have recommended getting a Defensive items instead of 5 offensive items; a Randuin's Omen would have done wonders to help you survive and to kite him out.
The big issue with that game was that your entire team dealt primarily physical damage. The weakness of all AD comps is that the enemy can just stack armor and health to deal with their damage; they don't have to worry about magic damage at all.
In addition, your team built near full damage with the exception of Alistar. You didn't have anyone to tank damage for your team or to peel for you.
he dwel id say dont focus him if he build full tank no dmg like that he probertly leveld his turtle to for the bigger shield so he should be doing less dmg try focusing down every squeeshy first and than all vs 1 the tank
I know right???? whenever I play sivir there is always this udyr whose build is
and mine is
and I can never beat him omg
on a more serious note, udyr is a great duelist. you probably can't beat him 1v1; your team just needs to cc him while he's attacking you
Get your magic damage champions to deal with him?
BotRK should have done something (being magic damage on % health and him having lots of health but no MR)... however otherwise I feel like you should have built a PD or Shiv in place of Ghostblade. The sustained attack speed and crit chance would have increased your DPS more than the flat armour pen, attack damage, and the active.
But really, either your team went all AD in which case they had a huge edge after champ select, or your magic damage champion should have eaten his 0MR alive... If you were all AD, you probably would have been better off having your team madly push lanes whenever Udyr wasn't around and try to race the base or something...
He would literally do no damage with that build. Other than that, he has left himself extremely vulnerable to AP damage, so have your team mates take care of him.
Sivir trades good DPS steroids for AOE attacks. Perhaps consider picking another ADC if you hate going against tank compositions. A Jinx, Kog'Maw, Tristana, or Vayne would shred that Udyr in seconds.
use a pillow.. he will stop punching your face and lay there laughing.. its the only way.
Last whisper and bork of the ruined king.
I think the biggest problem was that you had no magic resist to counter his thornmail. You ended up with enough armour penetration that you aren't straight in "kill yourself" territory, but with only BotRK for lifesteal and no magic resist you'd still deal an awful lot of damage to yourself. I'd swap the berserker's greaves for Mercury's Treads because you already get decent attack speed from BotRK and the Yomuu's active.
Also, if someone else on your team had a Black Cleaver I'd swap that out for a Bloodthirster or a defensive item. Sivir's for applying BC stacks though so it makes sense for you to be the one to carry the cleaver.
hes basically a stun bot with mostly manageable damage with that build
what you want to do is try to get your team to peel for you
if their team is pretty fed and your team can't save you... well.... haha
otherwise i would expect udyrs to actually get the new devourer items its strange he went cinder
swap cleaver for 
When he runs up to stun you, make sure to spellshield it.
Try to get out of range to force him to use randuin's active. Wait for it to run out in the next two seconds (I know...), then use BORK's active to slow him down and speed you up. Throw a Q when he shields during your autos.
You know when to ult.