Hi there, im Jagsan and i wanna share my opinion building Mundo oh, and its mine first guide
It's still incomplete, so dont be angry, thanks )
P.S: Feel free to comment, u r helping by that, sorry for my english :)
There's bug that Magic resist is showed about 83, but it has to be 150~

Everybody has its strength , but and its weakness.

Pros:

Tank. U can sustain some damage.
Anti tanker skill. Your q does damage per enemy's max hp.
Slow. Again your Q
Steroids. Click e and u have some damage!
Nice ulti. fast hp regen.
U dont need mana!
he is mad scientist who loves pain

Cons

Ignite is your biggest enemy.
U have only one cc
Leblanc is your worst dream. She can kill u in 2 seconds.
U have no deffensive skill
skills cost Health

Quintess and seals. Why %hp? u would say that flat hp runes is better, and seals for armor is better suited than marks. First, these r better than fla, cuse they give u hp when u buy hp and level up. With Jugernaut and runes, u will have 12% more hp. At end (without runes ) u will have 4005 hp, and these runes will give u extra 544 hp!
Glyphs: u r tank and no comment.
Marks: u need armor!
Optinial Runes:
Quintess:hp/5 sec, magic res, armor.
Seals:
armor,hp and hp/sec
Marks: ad, aromor pen, magic pentatrtion( why mag pen? Cuse your q deals more dmg! )
Glyphs: magic re per level

About def masteries i will not explain, its easy why.
Off masteries. Free ad? gooo. Exhaust upgrade? Again good. U need some CDR. But why magic penetration? I said earlier that this let u damage them more.

Cleanse. Its good thing against ignite. this saved me about 4-7 times in match, so dont bevare of using your ulti!
Exhaust. Its for stoping enemy, running from em, and helping team mates.
Optinial Spells:
Heal. U can use it. But they have iginte.
teleport. Its good in solo top, but i dont use.
Ghost, when u dont wanna use exhaust.
Dont use these spells!
Flash. u r tank, not chicken, U cant help team with this, rly.

Working on that
U need insta gold hearth because it will give gold to u and hp, and can be upgraded Rounduins omen.
boots, u have decide, if enemy has loww cc and deals dmg with auto atacks, when u need ninja tabi, if they have more cc and deals dmg with spells , when u need mercury boots.
Nature force is making your regen more like garens passive, big, but your wont dissapear when r hitted.
Warmog, It's boost to your Atmas, Nature force and runes! GIves u gooood hp regen and its has stack passive, on kill u get hp and its regen!.
Atma's. it gives u crit chance, armor and ad!. its nice on mundo, cuse u autoatacking em when u r near em.
Phage. Its slow+ ad+ hp! its nice irem, u will upgrade it to frozen mallet!
Rounduins omen. It gives u a lot of thing, like slow, armor and cool passive!.
Now u can fully finish build with frozen mallet!
U can buy slighty different if u need, because its not stone, so u can change a bit!

Q. U throw blade to enemy and slowing it down. U do damage on his max hp, but it has min damage!
Its your harass tool, so spam it!
W. aura wich damages and gives mini Jugernaut buff< 10% to disabales. i know, thats nice.
E. Its mega ad increse! And u get more ad when u have less hp. Itls like trynda's Q, but it dont heals u :).
E. its thing why u r fightining in lane so long whitout recall!.
Max Q and R first. For harrassing and for regen. When u can max W/E. It;s your choose that is better, ad or minus time for disables.
Properly skills use will be writen in future

IT's not finished yet, but maybe it will help u.
P.S if u think that i didnt mentioted something yet, coment or pm me. And u can change litle items :)