Again 'Variables" are the answer
Make one called "specialPower" (call it anything you like)
Then for every enemy or special enemy killed add value to that variable.
enemySpecial destroyed ...add 10 to specialPower.
then have condition that is true only when the value of the variable specialPower is a pre decided one .
if specialPower>=100 then.....special move "kaboom" is active
Im sure there are many ways to approach this but this is pretty straight forward.