Update: A recent game update seems to have made it so explosives no longer contribute towards this achievement. I'm going to leave this solution up in the event it is ever changed back, but this method will no longer work. Some of the comments point out alternative methods. Another new update removed durability from weapons/items so you can use the Super Drill indefinitely. Using the Super Drill and cutting swathes across the landscape is probably the fastest method at around 22,000/hour.
They have recently updated this achievement to include tracking so you no longer have to view your progress through the Game Hub. As such, I was able to confirm that explosives do indeed count towards this achievement and are the much, much faster method of obtaining it.
Head to Fort Finch and there will be a merchant named Razzo in the basement of a ruined building near the landing pad. He sells explosives. I chose to use Explosion Bombs instead of TNT as you can just chuck them into blocks and they explode on contact, no waiting for detonation. Purchasing them from him is much easier than farming materials to craft them.
I would buy about 300 of them if you can afford it, otherwise, buy the maximum amount that you can afford. Don't worry about money as you'll actually be making it during this process. I wouldn't buy more than 300 a trip, either, as you won't be able to hold all of the dirt you'll create.
Now, head to another world and dig a shaft straight down using you pickaxe. Once you're down a decent amount just start chucking explosion bombs and creating tunnels. Pick up all of the dirt blocks as you go. Usually I could hold about 300 Explosion Bombs worth of dirt before my inventory was full, then I had to run back to the merchant and sell the dirt/buy more bombs.
I used this method for the final 50% of the achievement and gained about 15,000 gold in process during which.
You could probably grind this achievement out from scratch in about two hours. I would periodically take a break and look for new Event Quests that may appear, though, since you need 200 of them as well to complete the game! It helps break up the monotony while still earning progress towards something.
In short - use explosives to blow up large amounts of blocks at once. Collect the blocks (even dirt!) and sell them back to the merchant (Razzo in Fort Finch) for a profit.