I'm certainly not one of the code gods, but this is my take on it.
First, you'll need to make a hud for everything that will be displayed. 10.. 9.. 8.. 7.. 6 etc
Then you need to create a script that counts the time down. It can be done, but you must remember two things. (at least!)
1. Only one timer will run at a time. If you use any other script in the level which uses the timer, the clock will be reset.
2. In order to make the count down script with the limited .fpi language, (and I use the term language loosely, I really should say script) you will need many, many, many loops of :TIMERGREATER=[NUMBER]:STATE=[NUMBER],HUDSHOW=[HUDNAME], etc. And don't forget that you need to initialize each hud in the beginning of the script.
As for the kill counter, someone else has already done that. Look
here.
Best.
I'm sorry, my answers are limited. You must ask the right question.