This can be easily completed via the AR Challenge
One Man Army under the Batmobile Combat category which is unlocked early in the game. It is recommended that you load this up to make this trophy as easy as possible, although the same method can be applied to any area of the game.
A critical hit is attained using the Vulcan Gun ([cn_RB]), and getting a direct hit on a light tank's weak spot which is located on the top, near the turret directly where there is a visible glowing light. Hitting this weak spot will make the tank explode and grant the player with a "Critical Bonus" or "Critical Shot" indicator in the top left of the screen. This is a cumulative achievement, and will be awarded after 50 total light tanks have been destroyed via critical shots. You can rinse and repeat the AR challenge until the trophy unlocks.