This achievement is earned for getting a 100 hit combo. It's actually quite simple to do, and while you're at it you might as well get a 200 hit combo to get more titles/grade.
*Minor spoilers beyond chapter 3 follow*
Basically all you need to do is spam Sylph with Milla against an enemy. Obviously this can only be done after you have access to Sylph again and it's easily done anytime in the post game. Also useful is the Minimum Damage skill. It's in one of Aifread's treasures in the Old Vicalle Mine. You won't visit the Old Vicalle Mine during the story, so you'll have to make a quick side trip. You can find it near Leronde. The treasure itself is located in the back of the mine and is rather simple to find.
Before the battle change all your teammates to Manual control and give Milla Minimum Damage. Next get into an encounter with an enemy. It's a bit easier if you choose bigger enemies like turtles. I personally did it against a scorpion. Kill off everything else using someone else. The game won't let you switch characters using L1+direction but you can go to the menu and switch with Start like past Tales games. Next switch back to Milla and push the enemy to the edge of the field and start spamming Sylph. Sylph gets 10 hits per use. Even if you run out of AC it will recharge fast enough that your combo will continue. The only other problem is TP; Sylph costs 40 TP per use. All you need to do is command someone else to use an Orange or Pineapple Gel when your TP is running low (Auto Items won't work since your team is all on manual).
I think this strategy works with Undine as well but I found Sylph to be easy enough. Please let me know if I forgot anything!