Well short answer: you shouldn't. However, I like playing Rammus a lot and found a 45% CDR build for him is great as it allows him to build full tank and cover all resists while making his W have a 1.7s cooldown. Which effectively means late game you permanently have 120 bonus armour and mr. Plus E (taunt) is on a tiny cooldown which is also really fun. You can reach 45% CDR through any items you like, this just seemed to work best in testing.

As a serious build this will probably only work through bronze and silver but my oh my is it a lot of fun.

Also someone please I really want a King Rammus account. Will suck ****s/give sheckles in exchange. Preferably suck ****s.

Nothing matters once you have 45% CDR. Everyone is inferior and you can easily 1v9 if needed. Just grind through the early game and enjoy the late.

That being said, Vayne, Amumu and similar %health champs can really hurt you late game so look to ban those if possible.