quote http://www.xbox360achievements.org/game/blue-dragon/guide/

Win a Multi-Monster Encounter against at least 10 enemies.

You need to get at least 10 enemies in your encounter circle, battle them all and win.

Where to do it: Southern corner of the Giant Snow Fields (top left corner of your map)
When to do it: Disc 3

There you will find robot spawning robots by a machine going round a crater and they don't flee either. Just run round till you have a group round you there. Get attacked? Simply flee and you get a few seconds of free time when you leave the battle screen to get in a good position. You can get in excess of 14 in a circle so its quite easily possible to do here. Keep track of your most in your Battle Record.