Get 25 kills with any of the hidden elementally charged Cleavers in Horde.
For a walkthrough of the Immulsion Cleaver on Jacinto, check the youtube video below. Once you get the elemental cleaver usable (rock bridge raised on Jacinto for instance), just kill enemies with it until it runs out of "ammo".
Note: It seems if you have the Comet mutator on and charge with the elemental cleaver and kill someone in the charge, it counts towards the achievement. It didn't take any of the Cleaver's ammo away and it was definitely counting towards the achievement for me.
You can't die and restart from current wave or the beginning, and you can't use it up and buy it again. This means you'll have to 'activate', use it, and quit all the way out and restart horde a few times or use The Maggot's tip above.
Note: Do not drop the cleaver, as if you do it reverts to a regular one and doesn't count anymore. This includes if you get downed, so be careful. Also, smaller enemies like tickers use less of the cleaver's 'ammo' than large ones like boomers.
Best done with at least one other person to help you. Grab the cleaver and target smaller enemies like tickers and wretches first and have the other person down and mark the bigger ones for you as the cleaver's "ammo" won't run out on downed and smaller locust and you'll able to rack up enough kills on your first try.
Almost worked it out for Cove, there are 10 big brown barrels that can be shot and release water. Sure it has something to do with the intact boat at Dock Spawn. Can't figure out the middle step. (Got bored) if anyone can work it out, enlighten me.
That YouTube vid above is great, but I have one tip. Don't put down the Cleaver once you buy it. Once its on the ground it becomes just a regular cleaver, and doesn't count towards the achievement. You can tell its still elemental if you have the yellowy emulsion glow around yourself. Once used up, it won't respawn, so just die, and start again at wave 1. I got 13 in one go with it, so should only take 2 run throughs if everything goes perfect, more if not (mine never goes perfect, grrrr)
Be careful with this piece of crap. If something stuns you, like a mauler attack, you'll drop the Cleaver and it loses its element (like Exaviour said), then you have to start all over. The Comet thing works, though.
What of those new maps are easyest to do this solo on? Seems like Jacinto or? How often can you buy the cleaver after "unlocking it in Jacinto? Did a few with a friend on artillery, but those thunderstorms+cleavers seems kinda rare sometimes :(