Contract: Any
Difficulty: Any
This achievement requires you to use the telescopic baton which is unlocked at level 17, and can be equipped via the inventory menu under the melee category.
This achievement can be done on any contract with police reinforcements, and is most easily completed with other players online in order to take the enemy's attention away from you.
Shield enemies are basically any type of enemy that holds a riot shield and they will spawn on any difficulty, although they spawn more frequently on harder difficulties. Although they spawn more frequently, they are also much harder to kill on harder difficulties, so keep that in mind.
During your many heists, there will inevitably be a police assault and these enemies will spawn. You will need to get behind their shield and start whacking them
with the telescopic baton until they die. It will take anywhere from 10-20+ hits to kill them, and you can weaken them with your pistol if you so desire. It can be very hard to get behind them if you are playing solo or with AI, so you may want to take turns doing this with a friend.