It's been done before
http://www.killermovies.com/forums/showthread.php?s=&threadid=299227&highlight=Magneto+vs+Green+Lantern
http://www.killermovies.com/forums/showthread.php?s=&threadid=355507&highlight=Magneto+vs+Green+Lantern
http://www.killermovies.com/forums/showthread.php?s=&threadid=391512&highlight=Magneto+vs+Green+Lantern
http://www.killermovies.com/forums/showthread.php?s=&threadid=395744&highlight=Magneto+vs+Green+Lantern