This achievement is for killing 4 or more enemies at once with Explosive Kunai.
This is easily done towards the start of Episode 3 of Nightfall as you're trying to sneak into the infirmary. Once you're introduced to the floating blue orbs, progress further and you'll find two groups of two enemies. Activate Kage (invisibility) and walk right in the middle of the four of them and quickly make some noise (left on the D-Pad) to attract them, then hide again. While staying hidden, equip the explosive kunai. Once they're all grouped up, aim at the ground and press Y to throw it at the ground. Once it's landed, hold Y. This will explode the kunai and kill all 4 guards, earning you the achievement.
Just to note that you cannot hit one of the guards with the kunai itself. If you do, and then explode it, it only counts as 3 explosive kills. All 4 guards need to be killed by holding down Y after the kunai is thrown, otherwise you won't get the achievement.
Here's a clip I made showing the process: