My partner and I were mowing down the other team, and we destroyed all of thier defenses, clear up to their citadel.
Of course, the opposition then just simply turtled at their crystal. During this time, they gained 5 kills, simply because it was impossible to kill them.
The game ended an hour later, when I was kicked out by the network.
In fact, if any team is losing by a large margin in any Slaughter-mode game and they simply do not want the opposition to win, they can just turtle at their crystal until they tire.
The only real way to fix this is to make the crystal at least temporarily destructable, preferrably easily destroyed. If any team has destroyed all the opposing team's defenses, then victory should be quickly secured anyway.