User blog comment:FairyRaven/Fairy Tail Chapter 470: Hybrid Theory, Review and Commentary/@comment-25111584-20160125131040/@comment-27281341-20160125142501

I believe that Acno didn't make a move during the GMG because he possibly was afraid that there was a slight possibility of him getting killed. 8 Dragon Slayers, 7 Dragons and (though he didn't know it) another 5 Dragons, including Igneel, might have been too much even for Acnologia.