This achivements is quite easy, but there are some things you have to keep in mind. to get this achievement you need to play the first 10 waves and you have to be in there from the start. Also every 10 waves is a boss wave which can vary between a reaver and the fearsome lambant beserker (there are of course al lot more boss types)
To make this achievement even easier set the difficulty on casual and put the following mutators on.
Instagib melee: you kill every enemy with a single melee hit (except for bosses).
Infinite ammo: well pretty much what it says (sadly only works on these weapons: lancer, retro lancer, hammerburst, gnasher, sawed off, all the pistols and the cleaver).
Super reload: when you make an active reload you get a 2x ammo clip (this doesnt disappear after a while) + massive damage bonus (you can kill a reaver with less then half a clip on insane).
Now just have some fun in horde and exterminate some locust.
Edit: since people wanted to know how to unlock these mutators ill add them.
Instagib melee: kill 200 humans in beast with a wretch, to get this the fastest your best bet will be to play casual solo to get the most kills.
Super reload: get the bronze master at arms medal (100 kills with each starter weapon), easiest to make a private match against bots on casual.
Infinite ammo: get 100 combat engineer ribbons (get 5 repairs in 1 deploy), see vid