Trek to Yomi

I managed to do it the first time. In phase 7, after a burning man runs past you, put the down to leave it on the correct weapon, get ready your finger on the RT trigger. As soon as he jumps to the floor below, press to shoot and kill the 3 enemies that are very close.

In Chapter 4 - you obtain the Ozutsu firearm. There is a great spot to set this achievement up right after. You come to a river and board a raft. There is a short cutscene at the end of ride and then you are surrounded on both sides by enemies. By this point in the game, you have the parry maneuver which places you behind enemies after a successful parry. Use this maneuver to position all the enemies on one side and fire away. It may take a few tries, as they don't always cluster close enough. If you miss the first shot, you can reload the check point right at the river.

In Chapter 6 you’ll come across multiple bridges around this flaming torii. (Japanese arch pole thing)

On your way you’ll encounter many undead enemies BEFORE reaching it. This is great because bridges force the AI to line up behind each other rather than fend off on the side. So you can guarantee to hit 3 at once if you manage to line up 3 enemies on the bridge.

Be a little patient with this. You can either block to slowly push them towards the bridges right before the torii or, if you’re really quick, you can PARRY SWITCH 3 enemies and have the follow you to the torii. They’ll be facing behind you so have 3 perfectly lined up for the shot and you’re golden.

Good luck!

Here is 100% complete walkthrough guide for every achievement and collectibles in Trek To Yomi: (Timestamps are in pinned YouTube comment)

I found a spot in Chapter 4 that was pretty easy to pull off. I reloaded my last checkpoint to make sure it wasn't too hard to repeat. You'll come across a small, arched bridge with a gunner and 3 armored enemies. Run up to the gunner and kill him. After that, use your parry skill to corner yourself to one side. Then make sure you have a good angle on all 3 remaining enemies and fire away. For reference, there is a checkpoint right before you reach the bridge. There's a slightly hidden path to the left that has two upgrade items. A bo-shuriken capacity increase and a stamina amount increase. To the right is the above mentioned bridge. It takes no time at all to reload the checkpoint and try again if needed.