FINAL FANTASY XV POCKET EDITION HD

I believe the description is misleading, it should say "Armiger" instead of "Royal Arms". I say this because I used the hell out of my Royal Arms ability and nothing, but then I use my Armiger several times and bam!, achievement unlocked. Who's to say if this is actually a buggy achievement or just has a badly written description. So just keep spamming your Armiger every chance you get. Hope this helps.