80% extra attack speed, 15% lifesteal.
+50% attack speed
Passive
Unique: Basic attacks deal 4% of the target's maximum health bonus physical damage on-hit,
I'm just going to say mundo has 5000 health, because i don't want to look it up, a yi/noct would be doing 200 true damage per hit, throughout the whole fight an extra 125 per hit with challenging smite, for the next 4 seconds
so 325 damage, per hit
and then master yi E or W does true damage equal to 18 / 26 / 34 / 42 / 50 (+ 35% bonus AD)
so lets just say he's built 200 bonus ad, his last 3 items can be deaths dance, some more attack speed, and then whatever
that's an extra 50 + 70 120 damage per hit
so 445 damage per hit.
the first hit from blade of the ruined king would hit an extra 8% of 5000 so 400
the first master yi hit, would deal 845 TRUE damage, not including his base attack damage, or a wits end
at 845 damage a hit, it would only take master yi 5.917 hits to kill a mundo with 5000 health. and well considering his build he would be dealing 2-2.5 hits a second, in 5 seconds he will have done 10-12.5 hits, and every fourth hit would equal 2 hits, so in 5 seconds master yi would dish out 12-15 hits
and it would take somewhere from 7-9 hits scaling down blade of the ruined kings 8% current health.
also of course master yi would be having lifesteal, so the thornmail doesn't matter at all, not only that but most of his damage is coming from true damage.
oh wait i wasn't even including his AD, if he has 300 AD, and Mundo is reducing 60% physical damage, yi actually starts his first hit dealing 120 + 845 = 965
Challenging Smite can be cast on enemy champions to mark them for 4 seconds, reducing their damage against you by 20%, and empowering your basic attacks to deal 48 − 125 (based on level) true damage to them over 2.5 seconds.
Unique: Basic attacks deal 8% of the target's current health bonus physical damage on-hit,
4 / 6.5 / 9 / 11.5 / 14% of target's maximum health every third E
btw if mundo builds 3000 bonus health and the enemy has 
, he can ult your mundo into his own team do deal 1600 damage on top of his base ult damage for free,
also how come you didn't calculate any grievous wounds into your calculations? you have a 6 item mundo, and somehow the enemy doesn't have 1 grievous wound item? anyways.
How would he die? Between Ult, Ga, and Sterak's rotations, the whole time his own damage plus Sunfire, and Thorn hitting enemys hitting him, combined with his normal Hp regen of 1.5% and 3 oceans which equal 7.5% = 9% amped 30% by Spirit Visage by 30% = 11.7% without even calculating Ocean Sol.
also how did you just add his 1.5% MAXIMUM hp regen, along side the 7.5% MISSING HEALTH regen? anyways.
So back to my origional question if the game is balanced, how exactly does this play out as balanced? Shouldn't Ocean Drakes be nerfed? I mean in this scenario You and your team just spent the time mana and ults just to take down 1 Mundo with 4 mini mundo's who all deal a hell of alot more damage than him coming in. How do you win that game, other than not letting them get 3 ocean drakes?
they give 2.5% of missing health per second, if you're missing 2000 health you're gaining an extra 50 health a second, per dragon soul. so in your case, 3 dragons, extra 150 health a second at 2000 hp missing, 75 at 1000 hp missing per second
first off lets just be clear, a burst mage and adc do not have 2000 hp to go missing. so you start at 1000 hp missing, gain 75 so then you go from 1000 to 925 and you regain 2.5% of that so 69.375, then the next second would be 64.171 that's 208 hp in 3 seconds, yeah it's pretty good, that's your reward for getting 3 dragons.
just like if you got 3 infernals, and had 300 AD, you would gain 12% more AD so 336, or if you had 700 AP, easy with 6 items and gathering storm, you'd gain 84 ap
no, ocean drakes should not be nerfed because one champion can abuse them, what is this logic, nerf the champion itself. and uh.. yeah since in your scenario clearly a tank mundo has 6 items, we're just going to assume everyone is level 18 full build, why shouldn't Mundo a late game champion be rewarded?
also yeah there's a number of champions who can shred mundo 1v1, well preferably a jungler with bloodrazor blade of the ruined king wits end
How would he die? Between Ult, Ga, and Sterak's rotations, the whole time his own damage plus Sunfire, and Thorn hitting enemys hitting him, combined with his normal Hp regen of 1.5% and 3 oceans which equal 7.5% = 9% amped 30% by Spirit Visage by 30% = 11.7% without even calculating Ocean Sol.
So to kill him without calculating resistances, or normal hp regen. you'd have to chew threw roughly 9k hp. That's counting Ult activation, Sterak's, and GA revive. But the time it takes to get threw that much Hp with good resistances would give hp regen what 20 seconds of 11.7% roughly adding on another 8k hp (at 10% though he has almost 12%) to get threw all without calculating ocean soul or the 5 stacks he's healing off of the stacked rav hunter and taste of blood. By which time his ult would of come off Cd, and you'll have to chew threw that, and by that time sterek's will activate a 2nd time. Of course now that your team has spent that long trying to take him down, there are 4 more of his team mates with the Ocean Dragons buffs to get threw as well.
if you want to continue this please do some recalculating BRUDDAH. he is not gaining 11.7% of his Maximum hp a second, he gains 1.5% and then 7.5% of missing + 30% from spirit visage
How do you win that game, other than not letting them get 3 ocean drakes?
well you could go into the game prepared for the worst, pick yi/vayne/sett, nunu if you want dragon control.. if you get dragons stolen from you then you got outplayed, and should probably lose.. shrug.
you can dodge. you can not let them get the 3 ocean drakes, you can pick an all early game comp, let the enemy have the first drakes or contest them, while making sure you take the rift heralds and push down a lane to their inhibitor by 15~20 minutes
https://www.op.gg/champion/drmundo/statistics/top/matchup
Ornn is broken at the moment, we all know that, and apparently has a 53.37% winrate against mundo
Enemies made Brittle icon Brittle by Ornn take 12% − 20.5% (based on Ornn's level) of their maximum health as Magic damage bonus magic damage the next time they are Stun icon immobilized by Ornn or his allies, consuming Brittle.
yeah I mean you're free to pick a late game comp consisting of Ornn Vayne, Yi, Cassiopia/kassadin/and some peel or pick support ( blitzcrank, 1v5 the enemy, with vayne/kass/yi/ornn they would melt)
Ornn can upgrade all of your items, he can deal 20% of mundo's Maximum health. so if 5000 health, Ornn automatically does 1000
also Ornn W = 12 / 13 / 14 / 15 / 16% of target's maximum health
Active: Ornn marches Unstoppable icon unstoppably in the target direction with 35% reduced Movement speed icon movement speed for 0.75 seconds.
Over the march he belches fire over a cone in front of him, dealing magic damage every 0.15 seconds to enemies hit. The final bout of flame will apply Brittle icon Brittle for 3 seconds.
so Ornn alone can deal 1800 to a 5000 hp target. easily. mundo doesn't have any dashes, and ornn has tonnes of slows/knockup, now if mundo is stunned by ornn, taking 1800, and a yi/vayne/kass comes in? he's FUCKED
also has a 41% winrate into 
Garen R
Active: Garen gains True Sight icon true sight of the target champion for 1 second, dealing them Hybrid penetration true damage after a 0.5 second delay.
True Damage:
150 / 300 / 450 (+ 20 / 25 / 30% of target's missing health)
but if you're picking garen, maybe you would've stomped mundo out, and garen would be 6 items, while mundo was 4
I mean yeah obviously in your world where Mundo already has 3 oceans and 6 items, and the enemy team comp is worse, then you don't win?
So back to my origional question if the game is balanced, how exactly does this play out as balanced? How do you win that game, other than not letting them get 3 ocean drakes?
this question is pretty uh.. simple minded, it's like asking " if the enemy is up 100 cs, with 2 items on me and he's level 16 while i'm 12, How do you win that game, other than not letting the enemy curb stomp you and gain a 2 item 4 level advantage? like how in the hell is that balanced? I need a chance to win too [sg-janna] "
yeah well. your chance was in champ select, during early game. during mid game, if the Mundo got to late game and the game randomly generated Ocean to be the third dragon and their team got 3 ocean dragons, and your team doesn't out scale him, and you have no way to deal with him, then yeah it is balanced for him to win....
and no the dragon is not the problem. cloud is not OP just because 
can already get a 15 second ult without cloud dragons, or because 
would have a 100 second ultimate without cloud
if you want to argue ocean being unbalanced, please use a normal example, I hope riot does not nerf a dragon because one champion exists that already has insane survivability without it, and now can abuse it because they get more. they can easily code in, Mundo gets only 1.25% per ocean. it's like asking if ocean is balanced because it creates more bushes for rengar/maokai
yeah the whole point of dragons was to add some excitement and variety, if mundo happend to get an ocean third, ( which i feel is the most common, but it may be a 25% chance ) and his team happend to capture them, let him have his op niche win, or nerf him not the dragon, but yeah there's a whole lot you can do to prevent him winning.
with 3 Ocean dragons and Ocean Sol with stacked Hunter and tast of blood in secondary. Primary grasp, Conditioning and Overgrowth.
or 
. To either increase healing or reduce the grievious wounds time on him.