A Plague Tale: Innocence

At the end of Chapter 9 when you are about to go into the University, you have 2 options

1) go to the right side where 2 guys come walking out, making it super easy and you can let them live or kill them, up to you
or
2) go towards the main door that has 2 shielded and helmeted guys who in front of the main door next to the carriage

Go to #2 the main door
I personally used a pot to lure one away and then sneaked up to the door past the 2 guy and clicked on the door and BOOM achievement

You HAVE to take down the 2 guards at the main door. The game will not show you the button to open it otherwise.

The other solution did not work for me, as the company may have patched this recently.

Either way, this is actually very easy to get.

As you enter the area, just ignore the 2 guys on your right side, and sneak up to the 2 (heavily armored) guards at the front door. Just whip out your Devorantis (helmet one) and shoot one guard and take him down with a rock right away as usual.

The 2nd guard is really easy if you go into this with a Somhum (the Zzzzz one). If you've got one ready, just walk up to him and let him try to kill you. That's it, you're done.

Walk up to the main front door and the Cheevo will pop.

easy method is to make a zzz potion and let one of the guards attack you then do the QTE and hit the Y button will put the guard to sleep. Then immediately create another zzz potion and repeat with the second guard, then go and open the door and the achievement should pop. The guards can both be aggro'd and you can manage them they are slow enemies.

Hey guys and gals!
Here is my 100% Achievement Guide for APTI!

At the end of Chapter 9, you have a choice of two ways to get into the University.
The right hand side where 2 guards come out of, but nobody is actually guarding the door.
Or the main door to the left, where 2 guards... Guard.

Obviously, choose the main door, sneak to the right hand side where you see a pot, pick it up, throw it as far left as you can and one guard goes.
The other guard stays and seems likely he will spot you, but stay crouched and go round the fire and he will not spot you.. (No killing and messing about involved)
Easy as pot pie!

If you prefer to see a visual for this, go to 1:00:20 in my Part 2 video, or if you prefer to see a solo guide for it, that is also below.

Hope this helps!

100% Achievement Guide Part 1 -

100% Achievement Guide Part 2-

Solo Guide -

What I did was take out the other two guards nearby first. Then got the shielded guards near the main door to spot me then ran and Hid out of sight

They are slow so are easy to out run. When one had their back turned, I used devorantis then a stone to finish them, then did the same for the other guard

It's quite easy to do it peacefully, without using any pots or potions. You just have to be swift.

Sneak behind the big carriage parked opposite the main entrance on the left. There are two spots between horses where only one of the guards can see you.

First, let the left guard spot you (half red bar). Immediately move slightly to the right and let the right guard spot you as well (half red bar). They will both walk towards you to investigate.

Quickly sneak to the right, all the way behind the carriage, wait until the second guard comes closer to the horses (so you will be out of his line of sight), walk to the door (do not run) and press "Y" ASAP.

Remember: you won't be able to interact with the door if enemies are fully alerted (full red bar).

The other guides are great, I just wanted to give a video of how I got the achievement because sometimes it's easier to get ideas on video rather than description.
This clip starts when I am up on the balcony right after releasing the cages of rats and gaining the attention of the soldier on the right side. It is possible to get around the soldier without killing him. Just let him see you and run up the stairs. You move much faster than him so can run down the other side and along the right side. The easiest way to get past the solider at the foot of the stairs is to use Devorantis and then a rock to kill him.
Once up the stairs, you can ignore the 2 soldiers on the right but there is a collectable in the barn. Don't try to run in before they come because they will see you and raise the alarm.
You won't be able to unlock the door and go into the building if any of the soldiers can see you or know where you are. You don't have to kill them necessarily. I killed one but led the other on a wild goose chase. Once I was out of his sight and shielded by the large carriage I was able to sneak up to the door and open it ending the chapter. As soon as you open the door the achievement pops.