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EDIT 16/11/2015 : a new patch now seems to correct this problem.
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EDIT 19/10/2015
Warning = It appears that a recent patch has made this achievement (and the "get run over by a car") unobtainable for some people. Unfortunately, no solution exists at the moment. We can only hope for a future update. Gameloft is still updating this game occasionally, contrary to what they said in a past news on their Facebook page.
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The Mall is the 4th world. The first opportunity you have to unlock this achievement appears on level 37.
The achievement says that you need to get run over by a bus, but you can simply hit the side of the yellow bus and it counts. Repeat 15 times and the achievement will unlock. It is cumulative, and you can restart the same level to get this.
If your are logged onto Facebook, you can unlock this achievement on a Win8 device and automatically unlock it on your WP (and vice versa).