Have a global variable for Timer and ButtonCount
Every Tick at dt to Timer. This will create a timer
Every time A is pressed, add 1 to ButtonCount.
If ButtonCount >= 5 and Timer <= 5 then
Spawn special attack
Of course the number 5 can be changed. With this example, A has to be hit 5 times before the timer reaches 5 seconds.
You will need to make the timer reset each time the button is pressed as well so you know if they hit A in the allotted time. Play around with it. I'm sure you will get it :).