This achievement can be quite troublesome. I found that the timing window for countering in this game can be challenging to match, and so keeping a great combo can be difficult. However, using my methods here, this achievement is actually quite easy. The only thing you really need to do, is keep jumping over the enemies you are fighting. As long as you keep doing this, your combo count should stay intact until you jump/roll away from them or are hit. Basically, the strategy is to hit an enemy 1-2 times, and then jump over them. You will need a large group to do this, depending on how much you have upgraded your combat strength.
There are a few really good spots to do this. I did it on the 8 or so enemies that you encounter when you first get inside of the Arkham Mansion. Other great spots to do this are *spoiler* the party of 16-20 enemies waiting for you near the end of the game, and the later stages (3rd or 4th) of many of the combat challenges. The easiest combat challenge to do this on is the Rumble in the Jungle ones, as the titan henchmen can also be jumped over if they are near, so you can jump over, punch, and repeat until they are down. Then you can climb on them and smack the minions around and get a combo that won't die as long as you're on the titan and don't get shot.
Using these methods, this achievement is actually quite easy.
One last note, there is an upgrade that gives you a 2 count instead of 1 for well timed hits. Using that, this achievement is made much much easier.