Infinite Undiscovery

To pop this achievement you'll need to be ambushed 10 times without your characters seeing the enemy before it attacks. So simply walking into a group of enemies with your weapon sheathed won't work.

The best place I found to grind this early on, was in Castle Prevant when you're with Aya, Rico and Rucha near the start of the game.

Go to the second floor, where the first chamber is with 2 friendlies and 4 enemies fighting (if you bring up the map you'll find it's pretty much dead centre of the level). To the east there's another chamber with 3 more enemies, and 3 archers to the north in the passageway leading to the darkened area. You want to kill all the enemies in this area - I let the 2 friendlies die since I didn't want them to interfere.

Once you're alone, sheathe your weapon and walk west back to the first chamber you came to. You want to walk only as far as it takes for the archer's passageway to disappear from the minimap, then turn around and head back. This means only the archers respawn, and not the guardsmen as well. Then simply walk to the centre of the western chamber, stop, and turn to face south - while having your weapons sheathed. The archers will shoot you in the back triggering an 'Enemy Advantage'; at which point you kill the enemies before doing it again.

Simple. It should take less than 15 minutes to get the achievement once you're there, even less if you already have a few under your belt. I know this method works because it's how I *just* did it ;)