Im sure there is a better way, but i know this way works at least.
Create a global variable and name it something like Random_Gen
This will be a number you will be setting to a random whole number within a range (I.e 10)
In the example I use "Spacebar" to generate a random whole number between 1-3 using the "Choose" expression
I then have a global variable which handles increasing the difficulty which in reality is just pushing the event check down a notch into a different set of layouts to pick from.
so you have 1-3 easy, 4-6 medium and 7-9 Hard
I used 9 levels just to get the point across, but you can set the "choose" expression to something like this "Choose(1,2,3,4,5,6,7,8,9,10)" (no quotes or you will get a string)
I hope this helps!