I tried doing your trigger. Basically, what it does is that every time your Hero attacks and it has Heavy Rain active, a random unit will be stunned. Is that right?
The conditions aren't the same because I don't know what HeavyRain is but I think this shortens your trigger while still getting the desired effects. But I don't understand the 200 range part. The ability only activates if the attacked unit is within 200 range of the Hero?
EDIT: I got what you mean now.
I tested it. It should work although 200 is practically the same as melee range. You have to be really close for it to work.
Again the conditions aren't the same as yours but the basic idea of what you want should be there.