In order to get this it's recommended to do the strategy for specimen GG:
It's worth noting this can be done on Easy difficulty as well.
The exact strategy I used was:
1. Place 3 satchel charges on the circular platform to the right from where you spawn.
2. shoot some rockets at the shield generator above to destroy it
3. Refill ammunition at the nearby crate for new charges and rockets
4. keep moving right to the next circular platforms and place 3 more satchel charges at each one, keep shooting rockets at the shield generators while moving.
6. Destroy the final shield generator.
7. Damage Nihilanth until the crystals show up, then blow your satchel charges. This will earn you the specimen GG achievement as well.
8. Keep pumping damage into the boss. I used the Gluon gun as I found reloading the rocket launcher was tedious, and kept running to refill ammunition at the spawned red crates. If done well, this should be well within 4 minutes.
It can be summed up as not backtracking, and keep moving right in a circle around the boss, while efficiently doing damage on the move.