You use variables with event actions.
On your shooter sprite put an instance variable.. say enemyShot=0
Now prior to or in your shoot event initialize variable using action set variable enemyShot=random(0,6), or enemyShot=(choose1,2,3,4,5,6)
Then use System compare instance variable and = 1 then action whatever, = 2, etc.. or in your example enemyShot<=5 regular shot >=6 special shot. (0 = 1.. so 1-7 is actually 0-6 in Construct2)