Easiest way to get this achievement is at the entrance to the robo base in the frozen area of the world map.
The fortress travels around a hill area in a circle, and leaves in it's wake lots of easy to kill robot monsters. With a little patience, and luck, you can get 10 of these (I managed 12 through sheer fluke!) monsters in your fight circle.
Very easy to kill.
Achievement unlocks after winning the battle.
Another location if the robots gets too annoying is on the 'Secret?' Poo Snake Island. Use garlic on a character to make monsters chase you, and you can often get 1 Poo snakes in 1 battle. Alos, this area spawns lots of rare poo snakes, Platinum / Gold etc, which award lots of gold/Exp and SP.
Also, as an axtra bonus, if you activate your shield on a character with massive MP, you can run around in circles obtaining as many SP as you like, just refresh their mana once it runs out.