All you have to do is you can buy any gun but keep the M14.

Now buy all the dooors to the power room and turn on the power, after that wait till the scientist comes and camp in a elevator (it's just easier).

When he gets close shoot him with the M14 2-3 or 4 times and he will die... and if you put the special code in the computer you'll get an achievement for 10 or 15 points!