Question Progressively harder enemies and another thing.

I'm just starting out in Unity and programming so I'm a bit noobish.

I'm working on a 2D clicker game in Unity but I want to make levels that the player has to progress through, like 10 enemies and then a boss spawns, beat the boss and you progress to level 2 and so on. The two problems I have is that I don't quite know how to spawn the boss at a certain amount of enemies, right now I have a script that randomly spawns a selection of 7 enemies over and over. I don't know how to make the enemies more difficult either.

As it is now I do have a script that changes the background after 5 enemies, obviously I was thinking of changing this to 10, spawn a boss and change the background when the boss dies and then make the enemies more difficult and the player progresses to level 2.

I'm at work right now so I don't have the code with me.

I know it is a lot to ask for and I am perfectly fine if no one wants to give me the code for said things but I have been googling and haven't quite found what I was looking for, so just a point in the right direction would be really appreciated as well.