The Hell's Janitor Medal is an achievement and objective medal that is featured in Halo 3. This medal can be gained by killing ten zombies in a row without dieing, it is available in all infection gamemodes.
In order to get this medal, you might want to rush less and camp more. Just be like the foe and stalk the zombies, don't use grenades to grab attention. Killing 10 of them in a row shouldn't be a hard task if right timing and weapons are used.