500,000 points is not at all hard to achieve: You can get more than that on the first stage alone. A general tip is to try to free as many prisoners as possible and staying alive until the end, as this will give you tremendous boosts to your score. For the first level, don't go into the submarine, and later go down into the boat with the prisoner. This route will give you access to 21 prisoners total, which will give you a lot of points.
When you get to the first mission boss, try to keep your vehicle intact until the end, as that'll give you another 300.000 points alone. Also, as soon as you see the big explosion that signifies a form change of the crab boss, stop shooting it and stay to the right until you've rescued a total of 10 prisoners on the bridge where you fight the boss. Then fire the last, few rounds at the boss to kill it, and the achievement should unlock during the mission end screen!