I am Split King a High Elo player with a Youtube Channel and Twitch account. At the end of season 9, I got to Grandmaster 400 points with my 3 Main Champions, Rek'Sai, Shyvana and Rammus.

I have played Dr. Mundo for a long time as well and he is fairly easy to play. I do NOT play Mundo above Diamond as he is far from a Meta pick at the moment. This might change in patch 10.3 where jungler xp is buffed which makes power farming a more viable strategy (Mundo farms REALLY well)

Mundo does however have a very easy time carrying games in lower elos and it's a safe pick if all you want to do is get to Diamond (or just plat I don't judge ;) )

Here is an example of how you can play Mundo with very little effort and still excel in Diamond games. (Note that I noticed in champ select that my opponents went almost all in on magic damage thats why I picked Mundo for a free win) Play Video

Most people think of Mundo as a Tank. But it is incorrect. A traditional Tank protects his team with cc or utility, Dr. Mundo has neither of these things.

Mundo instead protects his team by killing everyone surprisingly fast. (I guess you could say that killing someone is just a very long form of CC :D ) Mundo is basically just a tanky carry

Without damage Mundo would be useless as he would just be ignored by the enemy team even if he is quite close to immortal. It is for this reason that I believe you need at least one actual damage item to be effective on Mundo. My damage item of choice is Trinity Force. As of patch 9.16, Mundo has a 4-second cooldown on Blunt Force Trauma at rank 5. Reduced to only 2.4 seconds with full CDR (which this build has). Since Blunt Force Trauma is also an attack reset you can dish out some pretty extreme damage with a Trinity Force.

Against Heavy Magic teams Wit's End can work as your primary dmg item instead. The first items I build are however always going to be tank items as Mundo needs to reach a certain tankyness before he is viable. (basically your R needs to heal more than the dmg they can deal to you)

Start

Always go Hunter's Machete as your first jungle item it simply outperforms Hunter's Talisman every time.

You then either buy 3 Health Potion or a Refillable Potion. Mundo loses a LOT of health in the jungle since he uses his own health to fuel his abilities and speed up his clear so I highly suggest you start with 3 Health Potion if you are new to Mundo or the game.

Refillable Potion is only a good choice if you go Buff -> Buff -> Gromp to get level 3 from single target camps and the continue with a full clear. You sacrifice a little clear speed for a safer clear (you lose less health) and in return, you now have Refillable Potion to use on your next clear as well.

First back

Obviously you should try to spend as much of your gold as possible and you should spend it in this order:
Hunter's Talisman
Bami's Cinder
Skirmisher's Sabre

If you have spare gold you should buy a Pink ward and invest in a Rejuvenation Bead. Rejuvenation Bead sells for 105 gold so it only cost you 45 gold, think of it like a cheaper health potion that works over a longer time. (you will sell it the moment you need the money for something else)
Just make sure that you have a place in your inventory for Magical Footwear if you use that rune and is close to unlocking it.

[UNDER CONSTRUCTION]

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Link: Solo Queue Shen
Shen: A tanky split pusher with a supportive Kit. This guide can teach you a lot about split pushing and map awareness even if you don't want to play Shen.
Viable Elo: Mid to High.
Shen is a weird champion since he actually gets stronger the higher rank you are. Throwing a well-timed Shen R on a Diamond ADC does a lot more than throwing it on a Silver ADC. I do not recommend playing Shen Below Gold.

Link: World Famous Shyvana
Shyvana: A tanky Split pusher with very high sustained damage. I am disgustingly good at this champion. learning to play Shyvana does not take long. You can become fairly adept at Shyvana even with limited time on your hands if you follow this guide.
Viable Elo: Any
Shyvana is a very stable Champion. She is viable in almost all patches and all ranks

Link: Control Mage Teemo
Satan: A squishy Satanic Hamster that can control the battlefield with his shrooms. The map control is amazing. You can use it for vision resulting in a safe split push, or you can use it to place a heavy line of defense that makes it very hard for the enemy team to finish. A full build Teemo is also very scary. If you have sold your soul to the devil this guide is for you.
Viable Elo: Low to Semi-High
I can only play Teemo effectively up to about Diamond 3. Most players above that Rank knows too well how to abuse his weakness. I will however highly recommend using Teemo for climbing if you are below Diamond. You can climb fast and reliably with Teemo because he does not rely much on his team. Be a one-man army.

Link: THE Rek'sai Guide
Rek'sai: A void monster that burrows underground to stalk her prey. I got to Grandmaster with this champion and it's possible that I am even better on Rek'sai than on Shyvana. Rek'sai is extremely fun to play once you master her but it does take some time since she essentially has 7 abilities instead of the standard 4. You should use this guide if your main concern is to get as high rank as possible.
Viable Elo: Mid to VERY High.
You could still use Rek'sai in low elo with great success if you become good at her, but since she is harder than most champions to learn I would advise against this. When I play ranked on a new account I rarely start out by playing Rek'sai as there are other champions that can carry better with enough kills. You can, however, reach any elo with enough skill because Rek'sai is so extremely strong right now.

LINK: Rammus Guide
Rammus: A Tanky Tank. Rammus gives zero ****s because he cannot die. This guide is ideal for newer or low elo players since Rammus is extremely easy to play. You can learn the basics of the jungle without having to worry about learning a difficult champion at the same time.
Viable Elo: ANY (If picked as a counter) or LOW-MID (If picked every game)
I played Rammus alongside Rek'sai and Shyvana when I got to Grandmaster the first time. BUT I only pick Rammus against certain setups otherwise he is just too weak. The guide contains a section about WHEN to pick Rammus. Rammus is designed as a Counter Champion to AD.