Deep Rock Galactic has a trophy / achievement called "See you later, Detonator!" that requires you to kill a total of 20 Bulk Detonators. So before you begin grinding this achievement, you've got to learn what Bulk Detonators are and how to find them.
Bulk Detonators are called Glyphid Bulk Detonator in-game, and are a boss-type enemy that can be found in any region. It is considered to be very rare, and will randomly spawn in locations where bosses can spawn on mission.
This means that finding a Bulk Detonator has more to do with luck than anything else, and there is currently no known method to forcing them to spawn, or to grind them. Luckily they do seen to often spawn in pairs or up to four at a time, so you might not have to encounter them 20 times to be able to kill 20 of them.
Also keep in mind that these bosses are pretty difficult, and can easily kill you if you don't pay close attention. Aim at one of their three weak points on its back to kill it quickly.