OK! This can either be the easiest or the hardest achievement in ZOE2.
I'm gonna tell you the easy way!
After you beat the game normally you will unlock Naked Jehuty. Just start a new game as NJ on Easy difficulty. The game will be pretty effortless and if you skip cutscenes you can do it in 2 hours. Make 2 hours your goal so no messing around. Most enemy swarms can be killed like they were tissue paper with Phalanx.
Don't die at all. There is only one part of the game where this is even an issue and that of couse is the battle against possessed Ardjet. This is the only part I saved at other than the Big Battle mission. The timing here is difficult so if you accidentally kill her more than a few times exit and reload your save.
The only other parts to worry about are the 2 ranked mission. The first, the battle against the spiders is effortless. Just use Zero Shift and wipe the floor with them just as they are emerging from the tunnels. I didn't save before this battle but you may wish to. Make sure nothing gets destroyed and that you get an SS ranking.
Finally the big battle mission where you must protect 40 LEVs along side Vic Viper. This is easy. First save of course. Then Spam the heck out of Homing Missile. Literally that weapon will do all the work for you. Just keep an eye on the map and head towards areas with large amounts of enemies or where they are sending an SOS. If necessary use Geyser to help out the LEVs. I didn't bother playing medic myself, I helped them by killing all the enemies around them without effort. Reload if you don't get SS ranking. I got it on my first try. Not a brag... I'm not very good. It's just that easy!
After you beat the game it will rank you on your score. The criteria are:
Time: Under 2 hours
Enemies killed: Crapload
Highest attack chain or whatever it was called: pretty high during that one battle
Deaths: None
Saves 3
Mission 1 score: SS
Mission 2 score: SS
Your Score: At least S
Gamerscore: +30
And remember, if you want to vote this solution down please leave a comment as to why. Seriously don't be THAT guy.
EDIT: I've been told that this achievement and the one for A Rank do not stack. I cannot confirm this but I suppose blowing up a building on Mission 1 and letting a few LEVs die on Mission 2 and thus getting A Ranks will earn you A ranking instead. This is only an educated guess however.