I personally had issues finding and completing all of the Floyd missions. I found this GameFAQ written by Niveau helpful.Click here for guide.
I take no credit for this, guide written by Niveau on GameFAQs.
Furthermore, this achievement isn't glitched. Some people got the requirements wrong, flipped out, cried the sky is falling, and freaked everyone out.
This achievement requires you to get a BRONZE or better on all SIX missions. That's all. If you get a bronze or better in all six, you'll get this achievement. Some of the missions are required to beat the game.
Mizar's Palace - Automatically get expert for completing and needed to progress in the game.
SS Anubis - Gold required for earplugs, which you need to beat the game.
Final Mission after Final Boss Fight - Automatically get expert for completing and needed to progress in the game.
Just make sure you have BRONZE or better (not Merit) on the first five missions before beating the final boss and you'll unlock this.