The margwas are the large monsters with three heads in the map shadows of evil. They spawn in every couple of rounds and eventually you will start getting multiple ones per round. This is one way to do it, keep going in rounds until around round 14- 15 where two margwas will spawn.
To kill margwas you need to shoot all 3 of their heads. You can not damage margwas in the body. The margwas will start by moving very slow. You will see two of its heads/ eyes closed and one periodly open revealing a bright yellow eye. You must shoot this eye with a weapon enough until it dies, causing the margwa to freak out and release a puff of smoke (it usually only takes a few shots to take out an eye) The margwa now begins running and the middle eye begins openng and closing, where you msut do the same thing. Finally the margwa will begin running very fast towards you and you must hit its third eye. (Trust me killing margwas is much easier then you think if you never have) if you're at the round where two spawned just do this with both of them (also reccomend saving a zombie for the round) and bam you get the achievement.
Althought his method works, I have a secondary much easier method of doing this. It is easiest solo, but can be done with as many people as you want (just make sure the person going for the achievement is the one killing the margwas).
This can actually all be done on round 1, altought this is quite difficult to do on round 1 with less then 4 guys. Basically,when you complete certain rituals a margwa will spawn. These rituals being the one in the boxing gym in the waterfront district, and the burlesque in the footlight district, what I reccomend you do is save a zombie at the end of each round and open all the doors you can. You will need to open 5 total doors being:
500 points to leave spawn
1000 points to open the first door to footlight
1250 to open the second door to footlight
1000 to open the first door to waterfront
1250 to open the second door to waterfront
For a total of 4000 points.
As well, you will need a minimum of 2 beast modes (although you will likely need 3).
There aren't really 4000 points to be had by just killing all the zombioes on round 1 so, just go a few rounds until you have these points, probaably buy a gun at some point to.
Now just use your beast modes to grab the ritual pieces for the burlesque and boxing gym, and open both of them.
Complete the first ritual and a margwa will spawn, do not kill it or shoot any of its heads, now just run over to the other ritual site and do that one. just kill both margwas, easy, done before round 5.