While playing Voyage of Despair you will notice burning zombies showing up around the map. They blow up when you kill them causing a Catalytic Detonation.
You simply have to kill a bunch of burning zombies while they are in a group of other regular zombies. You will notice that they spawn much more frequently in high rounds approximately round 20+.
As far as I know it doesn't have to be 9 kills at once so you may get this over time when playing on high rounds automaticly.

To make this easier you might want to wait for a insta kill to show up but it isn't really necessary for this to work. Playing on casual with 3 bots also helps a lot.

Worth 15 Gamerscore.