Hey, I noticed on Orange Box you are missing "Hardcore". You know you can knock that achievement out in about 30 minutes right?
All you need is to do is become an engineer and build a Sentry and one of those replenishment things next to it, then have the opponent become a spy who will place a Sapper on the Sentry, while you just hold RT to hit the Sentry with a wrench.
This will kill the sentry and count as a kill on the opponent. It won't show up in your stats until you die, so say you killed the Sapper 100 times and then let the spy kill you, your stats will go up 100. Really quick method to get 1000 kills.
If you already know this, I apologise for the long post.