Get a combo of at least 30 during The Mercenaries.
I got this pretty easily in the second area on the Castle stage. Area 2 has 40 opponents. They throw a lot of enemies at you at one time so it’s pretty easy to get your 30 combo.
Set up your gun however you want. You just need enough ammo and power to kill quickly. Most likely however you are setting up for your S rank runs.
Area 2 starts with you at the entrance to the castle. Run forward down the hallway and then turn right to go to the room with the large staircase.
3 enemies spawn, but ignore them for a moment. Get the upgrade orb. Then start with the enemy on the stair case.
Kill him then the 3 enemies in the room. A lycna will spawn, kill him. Go down the stairs into the room that had the 4 statues in the main game. There are 2 enemies and a time orb. Take one out, grab the orb, take out the second one.
Now, facing the staircase 2 enemies will spawn in the hallway on the right. Take them out then go through the door on the left. You are heading back to where you spawned.
There will be a Soldat and several enemies behind him. After you have taken everyone out you will be at 13 kills.
At this point you are going to head outside. This is why it’s easy to get to 30. The game will spawn the 17 enemies you need outside and they will just run at you. You gotta move down the path and mow them down.
Achievement will pop right when you hit the 30 counter.