There's a couple of places in level 1 where enemies spawn until you progress. One being the place mentioned above. Also when the turrets are active outside the door, enemies continually spawn then too.
I managed to get 1.3mil points on level 3 co-op just but tearing through the enemies right at the end. Unending swarm with loads of enemies and tonnes of points. Put yourself as a Vanguard Veteran and combine that with a Techmarine or Sternguard and you'll rack up points in no time.
Theoretically you can get as many points as you want on any level with an infinite spawn, which I believe at least three of them have. It's just a matter of how long you're willing to farm points.