Only five gadgets can be quickfired -- Basic Batarang, Batclaw, explosive goo, shock gun, and freeze bomb. You get the freeze bomb after defeating Mr. Freeze, so you won't be able to get this until late in the game. Quickfire gadgets by double-pressing the number on the keyboard.
The best way to get this is to find one unarmed thug, and use all five on him. Go in this order;
Batarang to knock him back,
Batclaw to bring him in,
Shock gun to hold him there,
Explosive goo at your feet, then detonate,
Wait for him to stand up, then hit him with a freeze bomb.