Boss, signals?

[Esildor]

So I want to make a boss event that has like 4 waves:

Wave 1: 2 trash
Wave 2: 4 trash -- inactive until wave 1 dies
Wave 3: 6 trash -- inactive until wave 2 dies
Wave 4: boss -- inactive until wave 3 dies

I've made a boss that has adds spawn at certain percentages, and i've made a boss that has mobs come active at certain percentages.

But, I don't know how to make adds spawn when MULTIPLE mobs die, I think this is done with signals?

Can anyone explain to me how this is done, or, post an example of a script/scripts that I can look at?