Okay looked everywhere on this site for this can't find it anywhere.
Was just plodding along for Guild dungeon achieves with some peeps, and we discovered that in Strat (Dead Side) When after you go past the tower with the Spider looking boss, there is a set of gates that slam shut once you go inside, and aproxamately 30 - 50 critters spawn to attack you ..
These ALL count for the achievement.
I am aware it's kinda a pain to keep going in and out.. but Nice easy 2 - 300 critters per go (if you bring friends) .. Weeeee.