If you really want to bring back amumu to viable status, start by reverting his nerf on 7.8 and 4.20

7.8

Amumu Passive now true damage. Passive numbers down.

The function of Amumu’s passive (post-mitigation true damage) didn’t match expectations (true damage), so we’re switching to the latter and adjusting the numbers accordingly. Passive - Cursed Touch BONUS TRUE DAMAGE 15% of magic damage dealt, calculated after that damage was reduced by magic resist ⇒ 10% of magic damage dealt, calculated before that damage was reduced by magic resist W - Despair PERCENT HEALTH DAMAGE 1/1.5/2/2.5/3% target’s maximum health per second ⇒ 1/1.25/1.5/1.75/2% target’s maximum health per second

4.20

Amumu Less base armor, reduced W damage at early ranks.

With beefed-up health pools across the jungle and this little mummy dealing area-effect % health damage, Amumu's going to be a big beneficiary of the 2015 preseason jungle changes. We're taking preemptive measures to pull his clear speeds back in line, but we still expect him to be one of the stronger champions heading into this preseason. General ARMOR 27 ⇒ 23 W - Despair ADDITIONAL DAMAGE PER SECOND 1.5/1.8/2.1/2.4/2.7% of enemies' maximum health ⇒ 1/1.5/2/2.5/3% of enemies' maximum health

This champion was wrongly nerfed because there was a riot employee who though he could predict what was going to happen to the jungle meta so he decided to nerf amumu clear speed before even implementing the new jungle.

His W was hit at many occasion and it was his W that helped clear fast camps.