Gonna need to build mythic ASAP, build Leeching Leer -> Boots -> Riftmaker. Build Abyssal Mask next, it's pretty OP on you. Horizon Focus is next, unless you REALLY need bramble, then build it then Horizon. That's you're core, build situational items from there.

Assuming you do a 5 camp (skip krugs), you'll get about 750 gold by first back. This is enough for boots and an amp tome. if you have enough gold, buy a Blasting Wand instead. Finish whatever components you have, then go for Riftmaker.

Essentially, this build takes advantage of dumb calculations done with your passive and damage amp.

When Amumu does magic dmg with his passive and his core 3 applied, the magic damage is amped by 26.5% from AM and HF. This enhanced damage feeds into your passive, dealing 10% bonus true dmg. This feeds BACK into AM and HF, turning your passive from 10% to 12.6% bonus true dmg.

If you think I forgot about Riftmaker, I didn't. Riftmaker calculates its damage off of your base magic damage, dealing 9% true dmg. This is AGAIN enhanced by AM and HF. this turns Riftmaker from 9% to 11.3% bonus true damage.

These numbers are INSANE! dealing 23+% bonus true damage on all magic damage you do is unfair. But it gets better. Amumu's passive also works on your teammate's magic damage. AM also applies to your teammate's damage. If you think this might be broken, you'd be right.

Essentially, if your teammate does magic damage with your AM and HF applied, your AM buffs your teammate's damage by 15%. Your passive then does its damage. The thing is since you are doing the damage, AM and HF and Riftmaker apply again... doing 13.8% total bonus true damage...

Yeah, these calculations are a bit dumb.