I dont know if it's glitched or not but I would try and try and not get it until I found a little something.
There was a little trick that helped me get this and the achievement for the Mage and Scout on the second to last level in the Evil campaign where you have to clear a path for a battering ram. You have one of the huge orcs or whatever that follow you and you're in a castle I think.
I was at the final bit where you have to break a palisade right in from of a huge double door. The enemies spawn to the right of the door in a small room and they kept spawning and spawning and spawning as long as you dont destroy the palisade right in front of the double door. After I got this achievement I went to one of our flags to change characters and got the achievements for killing 100 with the fire wall and the scouts satchel bomb achievement all in the same playthrough.
Hope this helps.