Amumu is a great hero!
I agree that you will need a surprise element to be effective. Your regular attack does nothing, but tantrum does! As for summoner skills, if you like the golem buff go for ninja boots and smite, if not teleport and ghost give exellent results, flash is also a good option.
Amumu is not a tank as many people say, its more a second tank or support tank. It is best not to engage in battle with your toss and ulti. The better timing is when someone initiates and everybody gets ready to fight and push q,w,e,r buttons. Like 1-3 seconds after the initial fight has begon to come in and use your ulti, so they are of guard by the stun. As a eaction mostly they will start running, and thats exactly what you want
Item build: if you are the only 'tank' get warmogs first, no questions asked, if you dont, your dead more often then you would like. For a regular item build it depends on your opponents, there is no pre constructed item build for a tank hero. Depends what your up against. In most games, my standard build is as follows:
regrowth pendant (dont back before you reach lvl 6!), then chalis (never ever trouble with mana for the rest of the game), also get some boots (anti stun (also magic resist!) or ninja (if u have dodge runes or wanna jungle). after that its free choice, having a sunfirecape will increase your income a lot while killing creeps. Hit points are usually better than flat armor or magic resist. If you die from normal attacks, consider thornmail, if from magic, get more hp.
Amumu is a great support tank to play, you need a team for succes, but even if you dont get kills:
MONSTERASSIST EVERY GAME
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Strenght ?? Junior Member