Achievement has now changed since the previous solution.
You have to hit Level 50 on the Battle Pass. Well, at the time of writing, it is 45. Seems as if the achievement is tied to whatever level the unlockable character is. Although, this may just be a glitch for Season 4.
Each level requires 10,000 XP. You can get 9,000 XP from your daily quests alone. Completion of a game will give you several hundred XP too. With about 45 minutes of gameplay per day, you can unlock this achievement in 50 days.
If you look at your 'Challenges' tab from the menu, this will list all challenges that award XP. There is a cap on daily XP (3 challenges), but you can complete everything under Weekly, Event, Season, etc. I only aimed for daily XP challenges as 1 level per day is enough.
Remember, you only have until the end of a season to earn this. Once the season ends, you have to start all over again. So, do your best to get in 1 season!
Oh, and daily achievements reset ever 19 hours. However, they ONLY reset if you log in to the game. So, if the 19 hour timer expires when you are NOT playing, a new one won't start until you are in the game again.
Game is crossplay. You can play your challenges on ANY device (I played mine on Switch), but you have to be on the Xbox when you hit the target level, or there is a risk it won't unlock.