Well, you gotta remember
In general depending on how complex your waves are a decent approach seems to use groups inside the event sheet and enable/disable those as necessary. So you basically plan out what the next wave should look like, place all the spawn/setup events into a group, make it disabled and enable it when the time comes.
If your waves are super-complex it can get as far as making a system that, for example, takes something you have placed into a layout and stores all the important things into an array - then you save the array and use an opposite function to load the settings and create a new wave.
It's all going to take some work, either way.