SUPER BOMBERMAN R

No matter what, this is going to take hours upon hours. There's no way around it. That being said, if you intend to 100% this game, you should save this for last. You will get most, if not all progress towards this while grinding out the million blocks.
EDIT: if you use my million block solution you will probably still have quite a few bombs left over, maybe even close to half.
In the event that you finished everything else and still have some bombs left over, or you really just want to grind this achievement, there are a few things I would suggest besides playing the game normally.

1) Play through a campaign until you get 8 bombs in stock and get to the end of a stage where enemies don't respawn. From here, instead of finishing the level, you can sit here and use your 8 bombs endlessly to grind out progress.

2) If you have purchased the power zone(which you probably have to grind out gems), this map starts you with max bomb capacity so you can just grind it out here with very little setup, but this way requires 2 controllers.

Setup:
Rules: Battle Royale
Player: 4P
Com Level: Off
Sets: Your preference
Time: 9
Start Position: Fixed
Revenge Carts: Off
Pressure Blocks: Off
Skulls: Off
Special Abilities: On

Under normal maps, select power zone, then select whichever bombers you want, and bomb to your heart's content.

3) This method only applies to those who have a turbo controller, but it's probably the most efficient idle turbo method without getting too crazy.

*TURBO CONTROLLER METHOD*

Stage: Power Zone
Character: Plasma Bomber

(Both of these can be unlocked in the shop after gem farming for an hour or so)

Rules: Battle Royale
Player: 8
Com level: Weak
Sets: 5
Time: Any
Start Pos: Fixed
Carts: Off
Pressure Blocks: Off
Skulls: Off
Special Abilities: ON

After selecting Plasma Bomber, just add 1 cpu, set the others the none, and start the match.

You want to use the 8 player map for this method as the rows are a little longer, allowing for a few extra plasma bombs to be laid before dying.(normally you can only lay 8, but the plasma bombs go off fast enough that you will be able to place the full row of 18 bombs, since the 1st one will be exploding by the time you drop the 9th bomb.)

Turbo the A button, and keep something pressed up against the side of your turbo controller's stick so it is always moving right, or use a rubber band to hold it in place. This will ensure it drops the full row of 18 plasma bombs before killing yourself and restarting the match.

It's approximately 75 bombs per minute, so if my math is right, from 0%, it should only take 225 hours of turbo boosting to get the achievement. I'd recommend only going this route after you've obtained the 'Destroy 1,000,000 blocks' achievement first.

Credit for method 3 goes to MattusMattus17.

I must also add that me saying 50 hours using my method is a pure guess. I dont recall using any rough calculations to figure that out, but it should theoretically be anywhere between 50-200 hrs, since you never have to wait for match restart times.

Also, now that I think about it, the 8 player map is probably the ideal for all scenarios of grinding bombs since it gives you a little more room to work with.

***Turbo Controller Method***

Set-up a 1 or 2 player multiplayer game with the following settings:

Rules: Battle Royale
Player: 4P
COM Level: Off
Sets: 1
Time: 9
Start Position: Fixed
Revenge Carts: On
Pressure Blocks: On
Skulls: On
Special Abilities: On

Pick any Level & Bomber Types and when the game starts set controller 1 to turbo the A & B buttons. Doing this will result in Player 1 dropping a bomb down and killing themselves instantly, followed by the game automatically picking 'Battle Again' and repeating the process.

Following this method will also unlock: