Originally Posted by
Sprint
Several months ago someone with Gjallarhorn was asking about this over on the xbox.com ffxi forum. He wasn't able to get the achievement, and as far as I know never did. He just chalked it up horn not being a "weapon" and gave up. Seems there are other broken achievements as well from what I've heard. ToAU mission achievement wasn't working until last night, must have ninja fixed it.
I'd try sending a message to SE support through POL and maybe they'll fix it. Its more internal to the game, Microsoft just sets up the system, so I don't think Xbox support would be any help. Could also try a GM, but we all know how worthless they are.