If you wish to grind this out, I recommend replaying level 23 over and over. It's the level you're introduced to the bomb (Press
to activate before shooting). It's a simple level and you are constantly getting bombs. A few well placed bomb shots will end the level quite fast too. This is a good spot for beginners who aren't that far into the game yet. If you plan on going further, you might as well ignore grinding it since you will get it eventually by playing through every level.