Ant, I got mine the same as Ryan.
It's very easy in a Grifball match to get.
Zombies are the DXP this weekend. I'll be going for the 2 Mythic Achievements for this. They are both similar. In one you need to kill two Zombies, the other kill two humans (while you are a zombie). Simple!
Once I get those, I think I'll be set on my first account for achievements until ODST comes out.