If you are the kind of semi-beginner that plays Tremulous just to get kills, here is a way to boost your kill count.
When ATCS first starts, hustle your human bustle over to the entrance of the alien base. Sprint all the way. 2-3 dretchs will most likely jump at you in the first 30 seconds. If you are good enough to kill these, you usually have a clear shot at a granger meandering across your line of fire.
1 thing you want to remember is not to waste ammo. If there are 3 dretchs about to kill you, you don't want to be reloading. If necessary, back into the right or left corner.
You wont last forever in this position, but you might have a chance to kill several more dretchs/grangers. Not only does it add to your kill count, you get a ;lot of credits that can be used in later stages.
Hope this helps!