Spyke20

I'm not sure which guard you are talking but it must be one of these.
If it's the first guard of the level:
First, use the S.W.A.T. Turn to the right, then approach the corner with Sam's back to the wall.
While Sam is peeking, equip the Pistol pressing R2, then shoot the light above the guard and he should head to your position.
Use S.W.A.T turn to the left, then approach the guard from behind and press R1, you will knock out him instantly.
Don't forget to hide his body. For that, select the "Body" option from the Interaction Menu, when you are near him.
When you put him down, the Stealth Meter should start flashing indicating that you hide it well.
If you are talking about the guard with night vision googles, when you are looking for Ingrid, then you have to stay in the lights, using the spotlight to head to the other side of the courtyard.
Enter the door, go up stairs and neutralize him.
Have a nice game!