So basically I am a silver V pleb on 43 LP but my build has the intricacy and counter-meta thinking of a challenger. Basically, I'm averaging that out to mid-elo master tier. I have been a main amumu since I played this build that I thought of after playing a pacifist trundle game.

I thought to myself once, if everyone plays the meta... games are simply seeing who is better at playing the champions that they deem strongest. However, with the changes to Guinsoos rageblade and Wit's end I thought I would try out On-hit attack speed Amumu and the outcome has been interesting.

My friend told me that my itemisation does not synergise with Amumu whatsoever, but that is the wrong way to think about this. Let me break it down for you:
Kalista utilizes on-hit effects because she is an auto-attack based champion. As she is based around auto attacking it leaves that her only option of playstyle because people only THINK that's how she is strongest.

To put this into context; Amumu does not make much use of auto-attacks and is a CC heavy tanky AP jungler but if we itemise towards the playstyle of auto-attacking then it not only gives you the CC and natural tankiness of Amumu, but it also gives you that DPS and on-hit enhancement that you have always craved and the champion has always lacked. I feel like tunneling into a single playstyle gives you less flexibility and is therefore worse for your gameplay; so while this build may seem counter-synergising with Amumu himself, it is actually broadening the scope for mechanical outplay with your right mouse button and your abilities too.

I drafted up my initial build plan on notepad and had it on my second screen while I played, here is the image for you:

These are my successes with the Omemu build. The remake was because I said I was going full On-hit attack speed Amumu in /all chat and they tilted so much that 3 left.
Don't mind the losses, just think of the K/A's. In the 43minute game I actually went 2/2/11 but then decided to feed their lulu because I'm just that generous.

Pros: sick skins, Omemu is OP, kill everyone, good KDA, win game.
Cons: high amount of reports from team mates.