The 6th sniper is really hard to find, but he's in the very right skyscraper at the bottom behind the window (P.S. the guy who took Shorty is really hard to kill)
Flip with handstand doesn't work for me so I watched this video http://video.google.ca/videosearch?hl=en&client=firefox-a&rls=org.mozilla:en-US:official&hs=CZ1&ei=wUjBSbOVEaKomQe8yvStDw&resnum=0&q=shopping%20cart%20hero\&um=1&ie=UTF-8&sa=N&tab=wv#
3rd boss: when it disappears, go left for half a second then turn right and vice versa, then spam with gun/do melee combo (only hit 4 times with melee), last, you gotta repeat
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