The Billionaire achievement requires that you get a total of 1 billion studs in LEGO Worlds. This would be incredibly time consuming to do legitimately, so you need to employ some amount of grinding and exploitation of the game's features.

I have looked at other methods suggested and they were all too impractical and time consuming, and simply were not efficient enough to my liking.

Sitting in a pit and spamming the copy tool works, but takes a long long time, and the character moves too much.

Selling valuable items to the trader is nice, but they're not worth enough and as such also seemed inefficient to me.

So piecing together other aspects of other methods I derived the following procedure to hit 1 billion studs in a (mostly) timely manner.
That procedure is as follows:

1. Get the safety deposit box model.

• This is located in the cities, however if you still cannot find it, there is a quicker and easier work around to get it outlined in this video:

2. Create a new small world of any type.

• I used a world with very little terrain and mostly water, but it doesn't really matter.

3. Using the Landscaping Tool, with the add terrain and flatten option selected, create a large flat landmass to use as your base.

• In order to keep it all the same y-level, I found that holding down mouse1 and dragging until I was satisfied with the area was useful.
  
• Pressing E will make your template larger and make it easier to make this.
  
• You can choose to cover the whole world, but I did a sizeable chunk that was probably half or less than half the world and was satisfied.

4. In a separate space, place a bunch of the safety deposit boxes together.

• These dropped a decent amount of studs, about 15,000 each, if there is one with more stud drop, use that.

5. Using the Copy Tool, copy the bunch of safety deposit boxes you made.

• Saving it as a model will NOT drop studs, so you must copy a new selection each time
  
• Delete the bottom that has the terrain on it.
  
• The Copy Tool has limits so be mindful about how big of a selection you are trying to make.

6. Place your copied safety deposit boxes all over the large landmass you made in step 3.

• Try to use as much of the land as you can if you didn't cover the whole world.

7. Get out a vehicle or rideable animal and run into the safety deposit boxes to get the studs.

• Using a vehicle or animal is useful as the studs will automatically get picked up for you.
  
• I used Cole's Digger Drill or whatever but you can use any vehicle/animal that you prefer
  
• You may want to turn your settings down as it can be quite intensive given all the moving and broken parts
  
• Be sure to save regularly if you are worried about crashes from overloading the game and your computer with a lot of moving broken parts.

8. After you've destroyed all the safety deposit boxes you placed in step 6, repeat the procedure from step 5.


Each time I destroyed all of the safety deposit boxes I placed, I got between 150 and 200 million studs roughly. If you maximize your space and use a larger land area, you can get more studs before you need to place the boxes down again.

This to me was more efficient than other methods I tried. It will still take a while though, so put on some good music or a podcast and get grinding.