Same for me as well, only started using the gadgets near the end of normal campaign. Combat is soooooo much better when you get strings of combos using the batclaw etc. 
Do you mean just the basic take down? lft Ctrl and right click? Make sure you don't use it when people are around you, always try doing it myself, but get hit in the end before I knock the guy out and he gets back up

Do you mean just the basic take down? lft Ctrl and right click? Make sure you don't use it when people are around you, always try doing it myself, but get hit in the end before I knock the guy out and he gets back up
