Your playing Mundo a tank a very annoying tank that kills people. Mundo relies on his massive late game health armor and MR to tank damage in teamfights and take towers with ease. (not using ult)

-Mundo is a tank glass canon doesn't work very well
-Mundo can be very squishy early game but becomes a formidable tank late game
-Warmogs armor is a key Mundo item but you cannot get it in aram

Mundo is one of the largest tanks in League of Legends so you want to use that tankiness to your advantage.

In a fight Mundo actually works best NOT initiating but letting an ally purposefully take a bit of damage to distract the other team and get them to use high burst. Then Mundo runs in throwing cleavers at high priority targets putting on his W and finally activating his E you want to keep your E up by constantly reactivating it when they begin to focus you use your ult. Remember though in a fight use your E after using your first cleavers and activating your W this lowers your health allowing you to deal more damage in your basic attacks.

Mundo is a late game tank capable of dealing mixed damage. Mundo has high sustain and becomes nearly invincible when using his ult. Mundo is more of a passive poking champion early game but late game becomes a all out teamfighter. there are 2 rules when playing Mundo 1:dodge skill shots 2: throw cleavers in faces GG