You must get this achievement while playing with at least one other person, as your personal score in solo does not count as a team score. Nonetheless the map you choose is up to personal preference or the game, but I'd highly recommend having a decent team just so your chances of getting this achievement are better than with a bunch of randoms. There are waves of zombies that appear, just with no round indicator. It is important to know that the zombies you fight are gradually becoming stronger over time.
Purchases are free in Rush! You can purchase weapons, perks, and pack-a-punch (although double-packing is disabled) all without spending your points. Prioritize weapons and perks for close-to-medium ranges as that is where almost all of your zones confined to. Watch your down limit as you only have so many possible downs, but you can earn more by gathering enough points.
The achievement will pop in game as soon as your team collectively earns the 500,000 points. You also have the chance of earning the achievement for getting your personal score of 250,000, but it is possible to earn that solo if you prefer to work by yourself!