Reaching and surviving wave 40 on this map can be difficult. The way that I did it is to first get a group of guys that will join you and stick around with you in a PRIVATE Horde match. This is the best way to finish ANY horde; because you can set the difficulty to CASUAL. Other advantages are that you can also set the starting wave to the last completed wave you did if you didn't make it to 40 on the first attempt.
The enemies can spawn by the monument (Hammer/Mortar), both side wings of the main courtyard, the raised rear platform with the Sniper/Torque, and in the abbey (Flame/Boom). This all complicates things because they spawn in a flanking position if you just hole up by the main stairs. The most difficult waves are going to be ~36 and up.
My best advice is that on those waves you start to gather and place shields in ONE of the lower rooms (Boltok/Gorgon). You have an ammo box spawn, 2 main points of defense at each door, and one rarely exploited window to defend. With 4-5 friends split into 2 groups with some shield choke points this is the best place to make you stand through the final waves. You are close to the good weapons for easy resupply at the end/beginning of the waves. In wave 40 I placed a shield next to the sand bags while 2 others players held the rear. I also had a shield in my hand with the gorgon w/ zoom for easy head shots and tons of damage with the good cover for defense. We beat 40 on the first try this way, with multiple fails using other methods.
To Reach lvl 40 the best game types to play for exp are (% contributed by iiM SO N1NJA, thx)==
Guardian ~ 83%
Hope this helps, and good luck.