I found a really easy way to get this achievement and the 40x combo achievement in the Warren dungeon which you have to go in on the way to E Tower in the main story. You will be collecting 4 parts to build a flyer bot ,after gaining the second part from a chest it will show a door being opened to your right. When you go through this door you will see a bunch of small flying red enemies making a barrier so you can't go through to the green objective marker. Basically there are red enemies flying above this so as soon as you shoot a enemy in the barrier with red lasers they will be replaced straight away by one of the enemies flying above the door, so all I did was just keep shooting the bottom right two and they would get replaced with new enemies flying above. Keep shooting until you hit 40x combo earning you both the combo achievements! I have just tested this after you have cleared this dungeon in the main story and can confirm it still works!
Check out the video below thanks to Maka!