SUPERHOT

After spending quite a bit of time trying to unlock this, thinking that it all came down to plain dumb luck, I finally realized that the katana's on-screen reticle is all you need to pay attention to. Just carefully navigate until a bullet is within range, line it up with the on-screen "crosshair," and slice away. You'll have plenty of chances to use the katana throughout the game's main levels, but can also take advantage of the katana challenge levels afterwards.

Once I finally wised up and unlocked the achievement, I began slicing bullets out of the air with ease.

It takes a little practice but it's kinda easy.
Wait for a bullet to come closer and aim with the katana right on the bullet.

Save this one for the katana challenge. You unlock this challenge mode when you complete the game.

On the (fourth) level with the gun in the middle and two guys at the other end is the best place. Slowly move forward until you see a bullet heading towards you. Swipe and with luck you're done.

No real need to boost this though. You'll likely unlock it while hitting a shotgunner with your katana while he's shooting.

There is an extremely easy solution for this, the only problem being it takes a fair bit of time to work towards.

The first thing you need to do is complete the main story mode, so you can unlock challenges. Play through these challenges until you unlock the full stop challenge, in which time stands 100% still when you don't move. Play through the levels until you find a katana. Use this katana, and the time stopping, to line up the 'crosshair' (the diagonal line) with a bullet. Simply slice to slice the bullet in half, and the achievement will pop

Alternatively you could do the katana only challenge and hope for enough luck to perfectly line up the katana and bullet

Note: Don't attempt to slice a bullet that is coming directly at you. Move to the side so you don't risk being shot

After looking at the other guides I've found the best way to unlock it.
When you complete the game, you will unlock the "katana challenge", where you just have that weapon to several different levels to solve / finish.

On the Katana Challenge, Level 03, you will start on a large corridor, with two red guys at the other end, shooting another one in the middle of the distance (where is a place to move on the right).
Those two guys are with a head start shooting at you difectly.
The best way is to go a little to the right (because that guy shoot is more elevated than the left one) and after the bullet goes upon you it will be bigger. You have the line of the crosshair, move it to where the bullet is coming, thinking about when you do nothing the time spends very SLOOOOW. When the black graphic of the bullet is big enough over the diagonal crosshair, go for it and you'll unlock it.
I did it on the first try.