This playstyle of Mundo is definitely not for everyone and will most definitely get you flamed by your team. I recommend muting all here. What this guide will teach you to do is to EFFECTIVELY AFK farm.

Now you might be asking "Why AFK farm? Won't your team just lose and therefore you lose?". Generally I would agree with you, however Mundo is a very unique champion in the way that he operates. The whole "Mundo goes where he pleases" saying is very true and is something you can take advantage of. Still have questions? Let's go through my thinking and experience here. Mundo is a huge stat stick. What this means is he takes stats and uses them very efficiently and albiet very linearly. We can use this to our advantage though and have a linear game plan and playstyle ourselves as the player. The enemy team will almost always think you're weaker than you are, and when it comes time to team fight you will run them over 9 times out of 10. Due to how efficientally Mundo uses gold (stats) you will pretty much always be stronger than your opponents and they won't expect it at all. Will your team hate you? Yes. Until they see you run over the entire enemy team and have your first item at 9 minutes without a single kill.

With this playstyle, you should be aiming for a minimum of 9 cs per minute at the very very low end. 10 cs per minute is decent, however you should ideally aim for 11-12 cs per minute. This IS possible and isn't as hard as it sounds.

How do I achieve 12 cs per minute? This is from a mix of many things: Farming all camps on spawn, taking waves that are unattended, and stealing enemy camps. All of this can make you go beyond 11-12 cs per minute and will make you a monster.

The entire goal of this strategy is to spike really hard in the mid game. Once again, it's unexpected and no one will be reasonably as strong as you are.

When you spawn in, ward the opposite buff of where you start at 1:18. This checks for invades. I recommend generally starting on your blue buff. Your Q one shots single target monsters and you'll have your W for raptors and krugs. Hold your W for the tiny krugs. Full clear every game no matter what. When the time comes to take scuttle, remember that you should be out clearing basically everyone so go for double scuttle 9 times out of 10. When doing this, walk straight through mid and gank mid during that time period if possible. Don't force anything though, only go for damage on the enemy laner if it's free. Don't delay your path to scuttle too much if you can help it, but that doesn't mean you should ignore a kill. After this, reset and head to your next camps. Prioritize buying HP items (they convert to AD on your E passive) early. Personally I like building 2 giants belts; you will be extremely tanky. Keep full clearing and don't force ganks. Invade when you can and then go for dragons/grubs if you don't have any camps up. Only invade with prio. Don't be afraid to run at the enemy jungler with ghost and R. Make sure you're only using your R when you're low HP due to the missing health healing it gives. Side laning with titanic for example can be extremely good and will get you incredibly fed. You don't need titanic to do this, but it's especially good with titanic. When it comes to team fighting be sure to identify the biggest threat to you and do your best to run them down. Pay attention to where your team is and make sure you aren't going in 1v5 at all times.

This WILL happen frequently due to you not ganking. The goal here is to actually take advantage of the fact they're dying and take their CS. Your team dying here is actually a good thing. Why? Because them dying funnels all resources on the map to you. This increases your mid game spike and actively looking around to take advantage of this will skyrocket your power levels. A good thing you can do for your mental is actually look forward to your teammates dying and get more farm.

1. Start other camps early while finishing a camp or walking over to a camp by throwing a Q at them.
2. Make sure to use your E to hit tiny monsters for the AOE effect.
3. Use your W for the tiny krugs.
4. For wolves, auto each small wolf once then only target the big wolf.

Fastest clear video (not mine): https://www.youtube.com/watch?v=pz3mqGRcgFw
^ All clearing video credit goes to the video author on YouTube, not me!

Invading on Mundo is very simple. Check to see if you have lane priority. If you do, you can run into their jungle and take their camps without a care in the world. Most champions will never be able to catch/kill you once you're 6 so I really wouldn't be very scared unless it's a Jarvan or someone that can fully lock you down even through your passive.

I only ever stop farming when there's a cookie cutter way to end the game by team fighting for objectives. Bait fights at baron by starting the baron then turning on the enemies. This is one of the most consistent ways of ending a game. If it's still looking very difficult to end, keep farming as much as possible and don't be afraid to steal resources from your team if it's reasonable. Don't do it to the point where they'll probably tilt, but if they're far away and it's only once in a while you'll usually be fine. Just use your head and you'll figure it out in the moment.