To be honest, i'm bronze one so choose to use my guide or not, but i'm telling you from experience that this guide will help you with your own sad mummy adventure. Go find some friends!

This is a tanky build as obviously as it could be. This actually works well on amumu if your
just starting out, you can be more of a front line for your team and do some decent damage regardless of any items, but you can always build more damage later on.

This is what most players call grenade amumu. Basically you blow the enemy team up but you most likely will die anyway, hence the for mentioned ''grenade''

Just simple here, ionians for the extra CDR which is great on amumu, merc treads for the reduction on the cc and Ninja tabi for the extra tank stats that the enemy team will love

Just a simple ability order here that I usually go. W is gonna be good for AOE clearing and so is E so max either one first (Personal preference) I just max W first. Q is your main engage skill shot, if your **** at landing skill shots, max this one last as it will not matter too much if you miss as the damage output wont be too great and mana cost will be lower

Your ultimate is an amazing tool of mass destruction for your team. Go all in on a team fight with Q have W up and Ult, not only doing nice damage but also stunning the enemy and your E is gonna do nice AOE damage onto the enemy team, the burst is amazing.

Amumu's ganks pre level 6 are not something to be proud of, with little damage outside your ult your not gonna be to useful and may end up feeding a kill to the enemy. My advice, farm until 6 and gank when ever your ultimate is up. Whenever its down continue to farm but dont be blind, have map awareness and if an enemy is diving a tower, help them out, the stun no your Q is still decent enough to help your laner