Use every Freeflow Focus gadget in one combo
There are five Freeflow Focus gadgets you can use. The button prompts are:

Batarang (L2,L2)
Explosive Gel (L2+[], repeat to detonate)
Batclaw Slam (L2+TRI,[])
Concussion Detonator (L2+CIRCLE)
Glue Grenade (R2,R2)
The earliest you can do this is upon unlocking the glue grenade. Find a good amount of enemies together for this. Start the fight, then go through the list doing these one at a time, with a normal strike between each one. Be careful to not get carried away either and still counter and dodge to avoid getting hit. Make sure you are accurate with your gadgets, especially the explosive gel as if you miss you break the combo and have to start over. If you fail a couple of times in a group and there are too few enemies to get all of these in one combo, consider abandoning that group and looking for another. Once you get all of these in one combo this will unlock. This took a few tries for me.