I got this on the Combat challenge Sewer Bat (Extreme), playing the first round.
I avoided all advanced moves, sticking solely to attack, stun, counter and evade. The reason is that using takedowns and/or throws negate your critical strike, causing you to miss the mark. When I did a round and culled the herd with two take downs, I ended with a 38x combo, while sticking to my mentioned method I finished with 43x.
This will take a lot of practise and patience, as well as some luck.
EDIT: Some people seem to have problems getting this on round 1, but manage to get it on round 2. Unfortunately, I don't have the game anymore to re-visit my solution, so I cannot verify this.