You can do this via changes to your enemies' death scripts as well as a global script.
Change your enemy death scripts so that they increase a global variable by 1. That means your global variable will be holding how many kills the player has made.
Now, in your global script (or a script that is given to an ALWAYS ACTIVE entity/light/zone) have it display the global variable next to a "Combo" HUD.
If you want the combo to reset if the player doesn't kill an enemy within a certain amount of time, you can do this:
Change your enemy death scripts again so that they reset the global timer each time.
Now in your global script, have it check if the timer has reached a certain limit. If it has, then reset the global variable to 0.
How many times did I say "global" in this post??
*counts them*
....
I said "global"
10 times.
The one and only,
