So me and my brother tested this in the bloodbath arena and it works a treat.
The zombies DO NOT have to be alive to count towards the 10, so rack up 10 dead zombies in a small space and through your molotov, remember you can kick the dead bodies to where you want them.
Here's a video of me and my brother doing it in the easy arena.
Sol76 Added: Just to add , you can also do this in the campaign , however , in your quest to gather 10 zombies to kill , do not wander to far from your pile of dead zombies , because they will likely disappear. Try to gather as much zombies as you can and be ready for a chaotic slugfest. After that kick them so they are close together and throw a molotov in the middle. As soon as the fire ignites , the achievement pops.
A very good spot to gather them is on and near the circular restaurants on or at the beach near the lifeguard tower. You can do this as soon as you get your first molotov.