Combat Advantage is a core gameplay mechanic. By sneaking up on enemies from behind or stunning them with a weapon strike before beginning combat, you can receive various bonuses at the start of the battle, such as more turns before the enemy acts, or minor CP recovery. Similarly, allowing an enemy to sneak up behind you will give them the advantage.
In most cases, once an enemy notices you, you will be unable to circle around behind it, making it much better to focus on striking the enemy avatars on the overworld before starting combat. Once struck, you do generally have enough opportunity to circle around behind an enemy again.
Just get near the enemy and press
to strike. There are 3 possible results when you strike an enemy, which lead to the different kinds of advantages depending on whether you approach from in front or behind the enemy to begin combat.
1. The enemy is pushed back but not stunned - no advantage from the front, Single Advantage from behind.
2. The enemy is partially stunned and encircled with paralysis-lightning - Double Advantage from the front, Triple Advantage from behind.
3. The enemy is fully stunned with stars spinning around their head - Triple Advantage from all sides.
You are more likely to get better stuns if you attack from behind and without being spotted, but head-on attacks once an enemy has noticed you still work. Laura and her broadsword are particularly adept at causing stuns on most enemies. You can keep striking an enemy until you get a stun if the first one just pushes them back.
Just do this 300 times and the trophy is yours.