Easy to Hard spawning and No same time Spawning

Weswog

How do I spawn actors and it gets harder and harder faster and faster? How do I stop my spawning actors from not spawning all at the same time so they each don't all spawn at once?
EDIT: I have tried random under the expression editor but eventually the spawn actors spawned at the same time

Cheers, Weswog

Weswog

There are four spawners currently and there is only one type of actor being spawned.

Cheers, Weswog