This achievement is much easier than it sounds. To get the S Rank, you need to beat the boss rush in less than 6 minutes (really 5:50, as the last boss has a ten-second-long dying animation).
For the tactics to use, check out this video:
Don't worry about the items or levels in the video; you're not here to challenge Benja's time. I followed along the best I could, resorting to hacking away whenever the enemy didn't die or my MP ran out, and breezed to a 4:20+ time. And that was at level 50 with no items used (apart from those which spawn between rounds).
So, yeah, actually a pretty easy completion if you know what you're doing. Hope this helps!