You can use the Batarang on one of them - remember which and then keep doing it to this one only - once you knock him out and there is only the other left to worry about, place 1 dose of explosive gel and when the Titan is near it, detonate and then beat on him for a bit.
Repeat until he is defeated.
The gel does NOT void this achievement, and as long as you ONLY Batarang'd the one/same Henchman, it will unlock during the cutscene after you've KO'd the 2nd one.
