I had a bit of trouble with this one, but with some patience and persistence, I finally got it. I'm going to include a quote from ManicMetalhead
below, since he has summed up the most informative strategy for Faces of Killed 3, but I have a few pointers of my own, as well (see below the quote).
All of the 'Faces of Killed' achievements rely on exactly the same principle, it's just a matter of the length of the killstreak that you build up.
From the Main Menu, look at the Movie Shelf and highlight the movie called 'Faces of Killed 3' (it's on the third row down). It may also be called 'Yo! Skullface!' depending on your game settings.
You'll be thrown into a mini-game (which is a good place to make some money as well, coincidentally) that is all based on your reaction times and a small dose of luck. You'll see a bar appear at the bottom and a segment of that bar will show up red. You simply have to press
when the arrow slight [sic] above the bar is over the red section.
Doing it once, will count as 1 in the chain, and you'll see a gory kill in the background (unless you're on PG settings).
Doing it 25  times in a row without failing will unlock this achievement.
There is also no timer for this, so you can take as much time as you want in between each kill. Getting up to 20 and 25 is tough, so will require some luck and focus. It helped for me to turn the music off while trying for 20 (or 25)  as well, since it got a little distracting while going for the very small red segment.
I agree about turning off the music. In fact, I muted the whole game and everything else around me, before I could get to 25.
Also, you'll probably figure this out, but here's how the bar works. The skull with an arrow sits above the bar, and it slides left and right. You'll try to push
while the skull/arrow is directly above the red section of the bar. Sometimes the red section is very wide, so it's easy. At higher streaks, it starts to get very narrow, making it very difficult.
Now the skull usually starts somewhere to the right and slides all the way to the left, where it stops for just a moment. Even if you're mashing
as the current kill starts up, you cannot stop the skull on this initial motion to the left; only after it starts moving back to the right. When it does
start moving to the right, it moves at a pretty quick pace, but also speeds up as it gets further to the right. At the far right end of the bar, it bounces back to the left without pausing at the right end (at all). As it moves left again, it slows down slightly, until it stops again at the far left. It will remain motionless for a short moment before it starts moving again.
The speed of the skull's movement may
vary from kill to kill. I couldn't confirm this, but sometimes it felt faster or slower, overall. This could also be an optical illusion.
Experiment to find your own best method, but here are some tips of things I tried:
Find a rhythm. Sometimes, you can get into a good rhythm and hit the red zone the very first time that the skull starts moving to the right. This is especially true in the early part of your streak. If you're lucky and/or quick to react, you may be able to keep this rhythm all the way to 25, but I doubt it.
Sometimes, even at high streak values, you'll get a random wide red bar (easy). Take advantage of the freebie, and relax a little, but don't lose your nerve on the next one.
If the red bar is small and very far to the left, it can be really hard to anticipate when the skull is about to move. In these cases, I actually found it easier to watch the skull go back and forth a few times, and then hit
when it's on its way back to the left. It's easier to get the timing right if it's already moving.
Take your time! If you can't find a rhythm, or you find yourself getting impatient and making mistakes, remember that you can watch the skull slide indefinitely. Stop for a second, breathe, blink your eyes, and then get back into the zone. Watch it move back and forth a few times so you can feel
the timing. This helped me tremendously for my final run at a 25 streak!Bonus Tips Kinkyboots569
I found if you watch the skull and press A when the skull goes red, my reaction times are a lot quicker than just watching and waiting for the arrows to cross over the red bar KANKURO KUDO
advise i found cover up the top right of your screen, watching the number total doesnt help. take a deep breath and keep trying took me ages to do but glad i did it in the end Dr Bullard
Took me absolutely ages to finally get this. My main technique was to try and hit the bar the first time the marker went from left to right, if I let it bounce a couple times I’d lose my rhythm. As it’s been mentioned, my top tip would be to cover the counter in the top right corner. I didn’t think it was effecting me but I’d say around 3/4 attempts after doing so I managed to finally nab it.
Message me or reply with any questions. I'll be glad to help!