King's road may be the hardest difficulty but this achievement can be done on Lorien which is the shortest dungeon. Basically you want to get combo around 45-50 to ensure SSS rank, this is easily achieved by a single combo into a launcher (, , ) then doing the same to another goblin so basically there is always at least one goblin launched as the combo will be going until the last goblin gets up. I have also noticed that time doesn't factor in ranking but use of healing items might so try not to get hit that often and you should be fine. Can be obtained with a party.
Note: If you're playing with Slayer use Moonlight Slash as it's a better launcher. Put it on one of the hot keys so it's easy to use mid combos for example +. Remember to push twice while holding your hot key as this skill has two slashes and the last one is the launcher.
With Gunner it might be easier to use Mach Kick than launcher as it has better range but Goblins also get up faster. Try to end a decent combo with a BBQ or Gatling Gun to ensure SSS rank. (I don't put motions in skills because it's easier to use them from hot keys)