For level 50 you will need 8000 XP.
The fastest way to achieve level 50 in my opinion is boosting. Player 2 can surrender almost immediately after beginning the match. The game starts with 15:00 Minutes timer. The fastest I have achieved was surrendering at 14:57. Loading times are very short. I would estimate around 5 seconds from the winning screen to main menu. From setting up a new match to match start it were around 10-15 seconds.
I have done Level 1 to 50 in one session. It have taken around 90 minutes.
Surrendering will give you 50XP. So I have done 160 matches in 90 minutes.
I was a bit slow, because I was reading on PC while boosting with myself. 1 Match with setting up and surrendering have taken me around 33 seconds.