Well, if you really want to be adventurous you could learn ACS. With this you can have the death of any monster (or group of monsters) to trigger any event(s) you wanted. However, your map must be in Hexen format (Doom Builder labels it as "ZDoom (Doom in Hexen format)") and you would have quite a bit of learning ahead of you.
I personally think it is worth the time and effort to learn ACS. Almost everything you ever wanted to do in Doom is possible with ACS.
If you are interesting in learning ACS, here are a few useful links:
ZDoom Wiki
The ZDoom Wiki will be your best friend. It contains all the documentation on ZDoom and its editing features.
ZDoom Forum
The ZDoom forum is full of people who can help you answer any questions you may have about ZDoom and editing it.
A Quick Beginners Guide to ACS
This should help you get started in the right direction.
ThingCount
Once you get the hang of ACS, this is what you can use to create the effect you want in your map. It even has some examples!
I hope I was of some help to you. Good luck!
