This is really easier than I believed: I've tried many many many times, reaching max 30 hit combo before been hit... Also the Titan wasn't a good solution for me... I'm skilled enough in combat technique, but...
Well, here's the solution (really easy).
Done on Sewer Bat (not extreme) on 2nd wave, using just 2 button: hit and evade. You'll face a big amount of foes without knife or tube, so they're really easy to evade.
Well: face one thug, hit - evade - hit - evade - hit, hit (if you're not too crowded) - evade, evade (if there's more than 2/3 enemies), and so on.
No special combo, no batrang, or else... just hit and evade, no matter the distance, Batman will hit without trouble, and you can evade any time you need, because this wont interrupt the combo count.
You'll reach 40 before finish the foes.
Done on first attempt.