Knockout City

To unlock this achievement, you have to create or join a crew, and while in this crew complete 30 crew contracts. Each week it is possible to complete up to 9 contracts (and an extra contract each season), as they refresh every Tuesday. This means that you'll need a minimum of 4 weeks with most of the crew contract completed (e.g. 3 weeks x 9 contracts + 1 week x 3 contracts).

While this might seem a lot, many of the contracts are completed by normal gameplay, like "Win X rounds" or "Score Y Hits while within 10m of the opponent". Progression in these contracts is only earned in street play and league play matches, so you can't grind these out in private matches or training. Additionally, each week 3 of the 9 crew contracts are "Weekly Reward" contracts that automatically unlock after 2, 4, and 6 completed crew contracts, making it even easier. I suggest playing in a crew as soon as possible - preferably grouped with a crewmate most of the time as I will explain below - and you'll unlock this easily while going for the rest of the achievements.

You can see the current crew contracts by pressing in the main menu and selecting the crew section (car icon, see image below). While it's possible to complete some of the contracts solo, a certain amount each week requires playing with at least one other player from your crew, indicated by the description "While grouped with a Crewmate...". Therefore, it is highly recommended to find a crew that has many players, or a few that play regularly. The extra advantage is that each crewmate adds to the total progression of a contract. So, if you play grouped with two other crewmates (i.e. 3 in total), the contract "Win 150 rounds" actually requires only 50 (x3) rounds won. Contract progression is even shared among crewmates when playing separately, so it's possible that you've 'completed' a crew contract the next time you start up the game without playing yourself. Whichever way you decide to do this, after you completed your 30th crew contract you'll unlock this achievement.

TrueAchievement Crew
At the time of writing this guide, the crew "TA Friends" is used among TrueAchievement gamers that regularly play Knockout City. If you don't have friends that play this game, I suggest looking in this thread to join the crew (if it's still used when you read this).

Let me know if anything is incorrect or unclear, and I will try to improve this guide.