How do I get past the guard after Lascoyt has distracted the one outside?

Hint 1 of 5:

The guard must not see you.

Hint 2 of 5:

He can't if you stand either by the door on the left or in the middle of the
staircase.

Hint 3 of 5:

He is walking up and down.

Hint 4 of 5:

Listen carefully.

Hint 5 of 5:

You can hear his footsteps as he leaves.