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If they're *only* in iCloud then you'd be better of logging in to the iCloud.com web portal on a computer and doing it there. You can Ctrl+Click to select more than one at a time and you have a much bigger view.
 
I would personally do it on my Mac over my iPhone, but I have used Slidebox a few times. It keeps track of what folders and photos you've already managed and is a lot quicker than the photos app.

 
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