Warhammer 40,000: Space Marine

Our trick was stacking multikills with the Deamon Maul or the challenges in the early arenas. We went for challenges in arena 1 and 2 and ONLY capping points and kills in a set amount of time in arena 3. But we are all assaults with mauls and the Killing Blow perk so if you have Tacticals in your team, then feel free to get the headshots.

My team and I upon reaching the 4th area each time had 190,000 and
upon reaching wave 20 or the very end of wave 19, we had accumulated the 270,000.

This did not unlock for me until AFTER we had beaten wave 20. We had roughly 260,000 points when we began the bonus wave.
The bonus wave was pretty insane and we did not manage to beat it, but we were well over 280,000 points when we all died.
As a team we had two guys with Melta Guns and spent most of wave 20 hiding behind the machinery around the edge of the level picking off the Space Marines.

*Bonus Wave tips may be considered SPOILERS!*

The bonus round begins with 2 Ork Drednaughts spawning in the middle of the arena, followed by a whole bunch of heavily armoured Orks. Once again we ran the perimeter picking off what we could to gain points. The Drednaughts have a horrific mounted machine gun attack which did away with us very quickly.

Hope this helps somebody.