Quick explanation:
1. You need two friends one of them higher tier then yourself.
2. You ask him to host the game with other guy.
3. They have to go straight to first arbiter, after they kill the boss they have to enter next room straight ahead.
4. As soon as they load in, join their game.
5. You have to join before they kill the second wave at the top of the stairs.
6. As soon as the last enemy dies in this part of the room you will lvl up.
Another thing and this not always work you can leave their game, restarting your trials checkpoint. Then ask them to go back to lobby and load back to the room. Join them again and you can do it 4 times. Like I said not always work but you can always ask friend to run to this room from the start
source
Does it has to be two players?
It does not need to be two players. Can be just you and another person. Just tested it 3 times. Worked every time.
I need someone to do this, but who? ._.
Unless I'm doing something wrong, this is patched. Tried twice.