You will need 2 people for this and also need too turn on reflective damage.
After you have done that go to the building in Stalingrad with the large gun.
You go up to the top floor and don't go into the room with the gun but stay outside. You will see a rail.
Jump on the rail (walk to the wall and turn to were your back is touching the wall) and look forward (the opposite of the wall), you will see a blown up part of the building and a bit of it juts out of the ground.
You stand on top of that and throw the grenade around 4 feet next to the guy on the rail.
You count 1 one thousand, 2 one thousand, 3 and jump forward when you say three. It may take a couple try's but when you do you'll turn around and wait untill you heal.
Then you'll see wooden boards (remains of the roof), you'll ease out and jump on the one further right.
Once you get up there run and dont jump, just fall off into the snow and look right and you'll see a white space. You just need to run and jump off into it.
If you look up you'll see the map get smaller and smaller. This is how you know that you are in the right space.
Wait for 10 minutes and you wont be able to hear anyone. Your gun will bounce everywere and it will look dismantled.
This is where u can invite someone to a private chat and tell them to fly anywere they want as a spectator.
They can press start and choose the team (the team with the same guy down there) and they'll be able to spawn wherever they want as a spectator.
You will spawn in the same place you die so be careful.
P.S. you can chase people in spectator mode and you can spawn right behind them, that's my favorite