I Am Not A Pro Or Even A Fantastic Mundo Player But I Love To Play Mundo And These Are My Recomendations

Take Flash To Get Out Of Fights Or To Get In Range For a Cleaver Kill
Use Teleport To Get Into Lane After A Recall To Never Miss Out On XP Or Farm
You Can Also Take Cleanse To Help Get Ignite Of To Max Your Ult

Infected Cleaver - Pro - Good For Harassment,
Infected Cleaver - Con - Lots Of Health Lost If You Miss, Main Source Of Damage With Bad Cooldown For Teamfight

Burning Agony - Pro - Good Damage, Good For Farming
Burning Agony - Con - You Lose A Lot Of Health

Masochism - Pro - Big Damage Increase, Doesn't Cost A Lot Of Health
Masochism - Con - (There Aren't Any Really)

Sadism - Pro - Good Health Regen At When At Ability Level 2, Good To Get Away
Sadism - Con - Not Too Effective At Ability Level 1, Easily Countered By ignite

When Having An Even Lane Hit Minions With You Masochism To Get Good Damage For Last Hits
This Will Cost Little Health But Only Use It When You Have To.
When Having An Uneven Lane In :
Your Favor - Use Auto Attacks For Last Hit, Use You Cleaver And Masochism To Harass Down Enemies
Enemies Favor - Use you Auto Attacks And Use Your Cleaver To Last Hits And Harass The Enemy, Use Your Masochism To Get Last Hits On Higher Health Minions
When The Enemy Is Out Of Lane Ad You Are In Use You Burning Agony To Take Out Big Waves Of FArm Effectively But Dont Use It Too Much As It Can Put You At A Disadvantage