MISSION 7: A CRACK IN THE SLABIn "Crack in the Slab", right after you get the time travel device next to the piano this can be done very quickly.
Time travel through the door, and go straight ahead to the door on the other side. Peak into the past and wait until the guard turns his back. Then jump into the past to remove the blockage at the door, go through into the dining hall and instantly go back into the present to avoid the enemies.
Hide under the table and switch back into the past. Pick up the master key on a chair from under the table and you're good to go.