Shoot enemies and they burst into points. Try to collect the points while avoiding the enemies. The points lose value when they bounce.

Try to keep the enemy count low to avoid being overrun - but don't shoot too fast or be stuck waiting for ammo to regenerate.

Jump and duck with the up and down arrow or WASD keys.
Shoot with space, Z, K or L.

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The "Survivalist" achievement is really hard but it does work. A strategy is to stay near the spawn zone and shoot the enemies before they spawn. You can't get hit once or you have to restart.