Very easy to achieve early in the story.
I was roaming around London playing as Evie(Lv.2) and started a fight with a policemen(Lv.2). Just keep on bashing
, when the policemen's health bar gets yellow; counter his attacks with
. If his bar gets white stun the policemen with
. I was able to get a 47 combo level.
The video was taken a bit further in the story; showing a policeman(Lv.3) with a 42 combo level.
If you are having trouble getting a x40 early in the game, you can later on unlock the Unstoppable skills, these will aid you in achieving a high combo level. - docfester
Unstoppable I - "Increase the time before you lose your combo streak due to inaction."
Unstoppable II - "The first hit you take during a combo doesn't end your combo."