How would I go about enemy spawning? (C#)

My next step I'm going to be taking is enemy spawning, I'd love to take an approach where: Enemy type is predefined, but enemy level can be changed by a script on level load. The locations can also be static, but I'd prefer them randomized. How would I go about this? (I don't want a script written for me, just some steps, ideas, or a basic guide) I don't have much of an idea... If I instantiate with a level, I'd have to do it for EVERY enemy that I want, and I feel there could be an easier way... Thanks a ton!