You must play and win as the 5 Dastardly Bombers, which are also world bosses in the story mode - Magnet Bomber, Phantom Bomber, Plasma Bomber, Golem Bomber and Karaoke Bomber.
I did not have access to purchase these bombers from the store until I completed Story mode, after which they are available in the store for 5000 gems each, pretty pricey, which is what makes this fairly lenghty. While I have tried gem farming listed in other solutions, I found myself not getting gems after every match, sometimes several matches would go by before I could get anything, but I did find one setup that worked great, and hopefully it will speed up your road on this achievement.
First thing you must do is purchase the stage Power Zone for 4000 gems, there are no obstacles in this stage. You will also need Vic Viper as a bomber which I believe you can purchase for 10 gems at game start (this might not necessarily be a requirement but I feel will help speed up the process immensely, due to his dash special).
Start up your local Multiplayer with the following Rules -
Rules: Battle Royale
Com Level: Weak
Time: Really doesn't matter
Start Position: Fixed
I left the other stuff on as it really didn't matter.
Stage: Power Zone
Bomber: Vic Viper
Start the fight with just one Computer player, immediately dash down to the bottom left corner using the A button, and make your way to the right corner, where the COMP will have always dropped one single bomb in his bottom right corner before he runs off. You will blow up, and you will be awarded 150 gems. This should take no more than 3 seconds. Hammer the A button to restart the match with the same exact settings. Wash, rinse and repeat.
You can also work on your bombs achievement by dropping bombs on the way down and on the way right, I always ended up making it to the CPU's bomb to blow up from it before my own bombs would detonate. I was able to rack up enough gems in about an hour.
Negative feedback? Please leave a comment so that I can write better/more simple solutions in the future. Always open to constructive criticism.
Thank you and Good Luck!