One way you could do it is give your sword a variable, say "hit". Then whenever the sword is swung, if it hits something like an enemy, have events that increase the combo count by 1 and change the "hit" variable to true.
Then put a condition on said event where it only activates then "hit" is false.
Finally at the end of the sword animation have an event that changes the "hit" variable back to true. This way the sword can only "hit" something once per swing.