Record of Lodoss War -Deedlit in Wonder Labyrinth- (EP)

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!

Not sure on the exact timing for the S rank to pop. I got rank S at 5:45, assuming its below 6:00.
This one isn't as hard as the archery S Rank. Simply pop your large spells (the fire and wind ones at 50mp cost each) then use your stone spell inbetween. As soon as they spawn, pop the spell and keep hitting. After each boss make sure you grab the diamond and green potion (if needed) as they will disappear overtime.

If its a little too much for you to handle at the time. Either hold your spell book or pause the game to take a breather.