There are 5 tiers that you can reach by posting scores on the global leaderboard.

Bronze tier:
Bronze III: All players start here before posting a score on the global leaderboard.
Bronze II: Reach a score of 300 000 points.
Bronze I: Reach a score of 600 000 points.

Silver tier:
Silver III: Reach a score of 1 000 000 points.
Silver II: Reach a score of 1 600 000 points.
Silver I: Reach a score of 2 200 000 points.

Gold tier:
Gold III: Reach a score of 3 000 000 points.
Gold II: Reach a score of 4 000 000 points.
Gold I: Reach a score of 5 000 000 points.

Platinum tier:
Platinum III: Reach a score of 6 000 000 points.
Platinum II: Reach a score of 7 000 000 points.
Platinum I: Reach a score of 8 000 000 points.

Assassin tier:
Reach the top 1000th or better position on the global leaderboard.