Well the meaning of the variable in my example is not the same as the meaning you want to put in it.
My variable "mode" meant to differenciate phases in cyclic events (I could have called it "phase" instead of mode then)
You on the contrary want to make something evolve.
So, along with the mode (or phase if you want to rename it) variable, you need another one which will carry the meaning of "difficulty"
Then you can, as Kyat suggested, use this variable to increase the time of each wave like
atkDuration = 30 + 10*difficulty
30 would be the starting point
10 would be the increment of each difficulty mode (if you increment it one by one)