Warning: SAVE before you do this, because the achievement may not pop when it should, and missing it will require a whole new playthrough.
You get this after you've bloom'ed Shinshu field, and have visited the pyrotechnician. Talking to them and painting the bomb (a circle with a wick at the top).
You'll meet Bakugami, after this, the achievement will pop.
If it doesn't, reload your save and try again.