To get this achievement you need to get 5-6 kills at once 50 times.This is cumulative over all playthroughs so doesn't need to be done on your first run through.
The two easiest ways of achieving this is by attacking groups on enemies either with explosives or an upgraded shock blast from Dans Assault Rifle.
There are three good locations to do this where you get this type of kills easily on any difficulty. One is in the sewers chapter 2-5 when you face the monkey-like robots, then again when you face the zombie-like hollow children in chapter 5-3 and when you take control of the Howitzer in chapter 4-5.
I used the Howitzer method. When you first take control of it an Iron Whale transport ship comes in on the right hand side, it opens its doors and 6 assault mechs are just standing there, kill them, load the checkpoint then rinse and repeat until the achievement pops as it keeps count of all your kills. You can move forward to get a better angle and if they have managed to get out of the ship before you shoot they very kindly stand together for a few seconds.
Which ever area you choose, just use the above method.
The howitzer is also a good weapon to take out the four engines of the Iron Whale for that related achievement.