First off, it's very clear that you need to get a combo of 50x
Then get the "Critical Strikes" gadget , it gives you 2x if you use it correctly (just be patient).
I don't want to give any spoiler, so I'll be a little evasive sometimes :P
- For me, the best type of fight to get this during the game is when you face the big guys with hammers AND some thugs.
- Make the hammer guy dizzy and start the sequence. It won't kill it and you have 2 options: someone will approach OR the guy will try to hammer you.
- In both options, just press 2xA (jump over)
- If you come back to the guy, make him dizzy and strike him again, you'll probably get the achievement right there
***************IT MAY BE A SPOILER*******************
The train scene is perfect for that.