Yeah, since his passive reduces magic resist by a flat amount, Abyssal Scepter and Sorc boots would be nice. --- Voltron had cats for arms. So when he punched you, you got mauled by cats. - Cereburus Q Rhaeme made me lol irl --- lalak
You want a Haunting Guise/Sorc Boots and an Abyssal Scepter.
With those two, you are getting 20 Magic Pen, and you are also getting 20 MR Reduction. Combine that with Amumu's passive, and that's 65-75 of the enemy's MR that you don't care about. With the Magic Pen reds, you are ignoring another 9 of the enemy's MR. The 55 Reduction will make your magic attacks do MORE damage than they normally would against squishier champions.
Choose between HG and Sorc Boots. If you get the HG, you can get Swiftness Boots. If you get Sorc Boots, then you are more flexible and can get a Guise later if you notice the enemy stacking a bit of Magic Resist. Sorc Boots are also cheaper than HG, and you are going to get Boots, anyway, so it's even cheaper.
Amumu really guzzles mana, so he needs Mana Regen, so an Archangel's Staff is probably very important, so actually build Abyssal Scepter a little bit later.
A buddy of mine had a more dangerous and more unreliable build with Deathfire Grasp, Fiendish Codex, Sorc Boots, Abyssal Scepter and Haunting Guise. He fancied this build as a "tank-killer" build and it's easy to see why. There's alot of Magic Reduction and Penetration and he has just enough runes to hit the 200 AP mark for DG with his full item build, otherwise he just chugs a Blue Elixir if he needs the AP, and Amumu naturally has his percentage based toggle spell.
Abyssal Scepter is absolutely an awesome item for amumu. I build it as my 6th item when tanking as him, as it improves the damage of all of his abilities (and his teams abilities) and gives him the extra MR to tank. It's win-win-win.
Also, if you're going for straight AP, stacking AA's is more cost-effective and powerful than stacking zhonays, though you'll want one zhonyas in there. --- Just All Gamers Evolved OGL / Cal O #jage Wii Friend Code: 6214-3668-5353-3511