There is an easier way than these other solutions, and will stop you accidentally being killed or messing around with disguising yourself due to the sentry turret. This was my idea and worked a treat in a gaming session a couple of days ago. Start up a game as normal, and then one person selects a spy and one person selects an engineer. The engineer sets up a dispenser unit. ONCE THE UNIT IS BUILT the spy can put up to 4 sappers on the dispenser in a row while the engineer whacks them off with his wrench. Make sure you do not do this early as otherwise the dispensers health will still be low and you run the risk of breaking it and then having to kill the engineer so he will have enough materials again. After this, the health gets too low and you have to wait one second (literally). After this, the health will be full and free for another four sappers. For each sapper smashed, the engineer will get one kill. These kills are then counted ONLY when the engineer is killed. This method literally gets you just shy of 30 kills a minute if you do it right. (I timed it exactly). If the dispenser is accidentally broken, set another up and try again. The engineer can also hold or rubber band the right trigger to hit the dispenser with his wrench. The spy takes a bit more careful timing. For each hit made to the dispenser itself without a sapper, the engineer will increase materials in case of mistakes.
EDIT 05/02/2012: To secure the shortest time taken to get this achievement, (particularly if you are worried about making the mistake of destroying the dispenser early or the engineer running out of materials - this happened to me about 3 times total whilst aiming for this achievement, so it does happen!) instead of killing the engineer to get him more materials, you can use this method near the engineers spawn point so he can run in there and replenish his materials, therefore avoiding long respawn times (for mistakes) or just cutting down on steps taken to return to the boosting destination. (credit to Quarantane for this helpful tip/warning/alternative).
HOWEVER - you must remember to kill the engineer every so often because (as a reminder) THE DISPENSER KILLS WILL NOT COUNT UNTIL THE ENGINEER IS KILLED. This meant that any mistakes that I made were almost welcome, as it was another chance to secure my kills. If the game time runs out and the engineer has not been killed at the end, he will lose all kills since he last was.