Well, there is a theory going around:
Supposedly, if you copied all of the app's files (from using a file program on the computer like i-Fun Box or iExplorer), and pasted them to the desktop, then connected your iPhone to the computer, removed the app files from the app on your iPhone, and put your iPad's app files in place of it, this would theoretically give you all of the skins, weapons, and coins you had previously on your iPad, within your iPhone's Pixel Gun app.
Of course, this is highly theoretical, and I would only suggest this if you have nothing to lose in performing this experiment. That's pretty much the only solution I could think of that would work for iOS. With Android users can simply log into Google Play accounts and sync their data with multiple devices. Sorry.