This has to be done in the King's Road difficulty of any dungeon you pick.
The main concept of the ranking system is to try racking up a high hit counter when you do a combo attack and try to perform a high number of hits across all the rooms you visit in the dungeon you are currently in. At the end of the session, you are graded based on your highest combo and the rating of the combos you performed during the session. With a high enough rating and combo, you get a high rank.
You can do this alone or with a group. If anyone or all of you in the party manage to raise the combo hits high enough in each room, you will all get the SSS rank and the achievement.
The simple way to do this is to make sure you use abilities or attacks that can juggle enemies into the air and then hit them while they are still floating up there. If the hit counter still appears, you can keep hitting enemies in a group and raise it even more, no matter if in the air or on the ground. It's a good idea to juggle them up in the air so the rating in the end of the session gets high enough for you. Any character with the ability to do air juggling is great for this.
Here's how I did it:
I unlocked the achievement all by myself. I play as a LV20 Fighter, I picked the Lorien King Road. I made sure to gather plenty of goblins to either the left or right corner of the screen (I'm more comfortable with the right side), then I do two or three normal attacks on the ground before I do the uppercut ability, then while the enemies are still in the air, I attack with some normal punches on the ground, while still juggling the enemies in the air, and then I do another uppercut to continue the combo. I was able to get a 39+ hit combo (with a mix of 20-30 hits in other rooms in that mission, but you could probably get higher numbers than that), my rate was almost at 80% and I unlocked the SSS rank that way.
Happy Hunting, TA gamers~ :3