Nothing said there would be unfair teams, and nothing guarantees kills. Remember that it only counts for -your- kills. Your team might get all the finishes, or maybe it's split down the middle. It's highly unlikely to get all the kills for this achievement unless you're already nearly done.
Overall I don't want to get into a debate about a fun event that also serves to help players with a difficult achievement. There's a number of threads already floating around about the difficulties of certain achievements, which is probably a better place to discuss them.
If you don't wish participate, I can't force you.