This can be tricky as most enemies take more than 1 hit to kill, but there is a perfect spot to get this.
Quite some way into the story you will cross over to the icy island to the south of the map, Peaky Peaks
. From there, you can head through to the Iceberg River
In Iceberg River
, there are at least two very large, white slimes. When you hit them enough, they will splinter into 8 small slimes. Note that there are other large slimes which will only split in 2 - you'll know if it's the right one if it jumps up and does a ground-pound attack.Edit: I had to replay the game as Essence of Rusty glitched on me. The large slimes that immediately split into 8 small slimes may not appear until later in the game. I don't like this game nearly enough to test any further but if you can't find one, just keep playing through the story - they will be there towards the end of the game for sure.
It shouldn't be too hard to bunch the small slimes up - run away and they will all chase after you. Throw a regular bomb into the mob and the achievement should unlock.
If you miss or run out of slimes or bombs, just quit the game. When you load back in you will be at the start of the area and you can try again.
I did them elsewhere but this could also be a good method for getting the other bomb-related achievements: