For me, the easiest way to get sprees has always been with the energy sword, especially on Countdown, because it has so many corners. Against a poor team, you can often get 2 or 3 kills right off the bat by getting the sword, going above the doorway to their "base," and waiting for them to come out. Drop down if they are dumb enough not to check behind them, and there you go. And if someone on the other team happens to beat you to the sword, there are plenty of ways to take it from them, like the shotgun or concussion rifle. It's really easy to get sprees in objective games on Countdown, especially CTF, because people are often rushing to capture or defend, and they don't inch around a corner like they might do in Slayer.