Idea for full game. Aiming at the guard so that he runs faster might be copium or unrelated to aiming. Shtok suggested he needs to both see you shoot and see the nurse being shot.

Can't vouch for consistency of Yuki's guards, but if you manage to poison both, the rest is consistent. Open the door after she starts yelling for the 2nd time, so that guards outside don't turn around. Without opening the door you'll have to wait by the wall for her to slowly walk up to you, and i don't think in that position the kill can be seen, and certainly not seen when you lure her closer to the exit.