I struggled badly with this and the other guide did not help me.
I found this info and it clarify things but I still struggled!
*** Spoiler - click to reveal ***An imperfect combo breaker is when you see "BREAK" and you fly backwards.
If you do a perfect combo breaker, you see "BREAK" but instead of flying backwards you stand still and see something looking like a rainbow shattering. It has nothing to do with taking no damage.
You cannot block the first hit of a combo. If you block any first hit, it's just a deflection, because they haven't started a combo yet. If you hit block between the first and second hit, you can get a combo breaker. The difference between imperfect and perfect is timing, not how soon you do it. Figure out how to get a perfect break, then work on replicating the timing with two controllers.
Use the 2nd controller trick and set the options to 5 duels just to be sure and change life bar to maximum.
To make it easy you should use the right characters too! Block with Elfin!
Attack with Nemus!
I got the achievement after about 15 attacks doing that!
While when I used other characters I could not even get the achievement within 5 rounds.
Also best done in the interstellar level!