Beat Zone of the Enders- The 2nd Runner with a rank of A
This one is pretty easy. At least if you play on Easy difficulty. It will be harder on higher difficulties but this is a guide for getting a cheevo not rising to a challenge ;).
Anyways, do the game on easy, if you already have unlocked Naked Jehuty use it and unlocked subweapons. Speed through the game as fast as you can. I'm not sure if they count Cutscenes against you but if you've already beaten the game skip those (I didn't but it couldn't hurt). Without skipping any dialouge I got through the game in 4 hours. With Naked Jehuty I estimate 2.
Save as rarely as possible. I recommend saving only before tricky parts such as boss battles and the two Rescue Missions. If at any point you happen to die, exit out and reload your last save.
Save duing the two ranked missions. You need at least an A (what I got), if you get B or lower , reload.
Playing on Easy the game is so simple that this achievement should be fairly simple.
Please leave a comment as to why if you decide to vote down this solution. Thank You!
Edit: I have been told that this achievement does not stack with the S ranking achievement. I recommend going after this one on the first playthough and making sure to get A rankings on this missions and not S and getting the S ranks on your 2nd playthough. Possibly with Naked Jehuty.
Edit 2: Or you could just aim for the S rank, making a separate save when you past the hyperspace battle. After completing the game with an S reload the save before the final boss and die a few times or save the game a few more times. I have been told that you get ranked down for saving the game more than 15-29 times, That should do it. Let me know if this doesn't work.