Detonate Grenade is an epic quality (purple) quickhack. I was able to craft it after unlocking the Hacker Overlord perk in the Quickhacking skill tree which requires 16 Intelligence.
After that simply find a group of 3 enemies gathered close enough together to be effected by the blast radius of a grenade. Lower level NCPD events seem to be the most likely to have the ideal circumstances for the achievement.
Edit: Apparently you can farm the group of enemies near the Chapel dataterm in Pacifica in order to get this as a drop. Mentioned in the comments by NoHeroes94 and confirmed by MyDanishReality.Edit 2: Video is not mine was posted in the comments by J2B9, all credit to the creator.