Double Trouble - Defeat two Titan Henchmen at once.
This is essentially the same as fighting one Titan Henchmen, only there are two of them. The same tactics apply to them as the other Titan Henchmen, wait until they start to charge then batarang them and combo them until their health is down 1/3 and 2/3 of the full bar and then you can ride them and hurt the other Titan Henchman. But make sure to watch the other one and try to make them hit each other, as it will save you a LOT of time and headaches.
NOTE: Credit goes to the original video uploader, as it is not my video. I only posted it here to provide a solution to this achievement.