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From this site: http://www.candiedskull.com/crackdown/achieve.html
"At the Observatory in the Volk territory, there is a giant globe in front of the doors. There are some explosive objects at its base. Blow it up and the globe will come loose and roll freely. When it does, use it to kill 15 gangsters. To make this even simpler, the Observatory is the scene of the boss fight with Natalya Gryzunova. Grab the globe when you go there for this battle and use it to kill the gangsters until you get the achievement. You can throw it, or kick it into gangsters, either way it counts, but you must have at least a 2 star strength to pick up the globe."
However, Natalya Gryzunova switches between the observatory & the shopping district. You can find more info about Natalya & where to find her here: http://crackdownoncrime.com/ThisIsCrackdown/Volk/Profile_Gryzunova.htm It even has a picture of the globe on that page.