Easier way is to evade while fighting. It's perfectly doable during normal gameplay but except if you are in hard difficulty level, in-game fights allowing this combo are pretty far from beginning. So, just do it in any fighting challenge.
Basically, just evade (A+A on controller) an enemy and strike it (X on controller), and repeat until x40 combo.
Asylum is the first in the serie so Freeflow system is less "assisted" than other Batman games. So, you have to "target" the next goon after evading to allow Batman to correctly strike. If you don't, Batman stupidly strike in free space in front of him and break your combo...
Don't rush, combo won't break if you do it correctly, even if it's not quick. Take you time : evade, target, strike, evade, target, strike...