The description is not written properly, in order to gain the boost one must use 3 different ways of attack - counter, sword attack, sign, bomb or crossbow. Using any combination of 3 among them, except fast and strong attacks as both count as a sword attack, will grant the boost.
It doesn't seem to matter if the attacks actually hit or not, with the exception of counters of course.
The boost will reset after landing the next strike on an enemy.
The boost will not reset, when using signs except Axii, so it is possible to keep it as long as other signs are used and no sword attacks are performed. (this is most likely a bug)
However, alternate sign modes will reset the boost with the exception of Yrden
As with most stat boosting effects it only takes base damage into account.