One way to do it is to use a timer to track when the reset occurs. This timer module or this one should work.
Here’s the logic:
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When you first damage someone create the timer and start it.
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When you damage them again check to see if the timer has already been created. If so then reset the current time to it’s original state to refresh the combo timer.
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Set an event for when the timer ends which resets the combo counter and also destroys the timer.