Laning against Mundo, you can kill him surprisingly easily pre-6. Mundo comes close to the play style of a raid boss, and with that, he dies pretty easily during the early game if you have a good enough hard engage.
I suspect your main problem with Mundo is his tankiness; once Mundo's tankiness begin to scale:
-If you're a fighter or tank of the team, try to body block as many of Mundo's cleavers and protect the carries. Generally speaking, by the late game, the Carries are really the only ones who can phase Mundo. Because of how his W works, save any Tenacity weakened CC as it's not as effective against Mundo, although displacements (knock ups and knock backs) are still effective against Mundo.
-If you're a carry for the team, itemize for tank busting (
, 
). Your job is to make sure Mundo isn't sitting on you. Kite and let your teammates peel, and definitely do not overextend during a team fight. It's a carry who steps just a bit too far forward that Mundo will eat up and kill with ease if Mundo has his eyes on diving.