This achievement is
Overload Specialist worth 15gs.
For this achievement you need to use your special ability, Overload. Only
Engineers and
Sentinels are capable of getting this achievement. If your playing as a Soldier, you cannot make your squad mates do this, even if they have the ability. The power has to come from you for it to register. This achievement also registers with Multiplayer, So if you don't want to create a new character, then you can use it in Multiplayer.
Just use this power on 100 enemies that have shields and it will pop for ya.