I was able to do this on Heroic with a few skulls enabled: Fog, Eyepatch, Boom, Famine, Pinata.
If you finish the level around the 20 minute mark (I think I may have been around 22 minutes) you will get a 2X time multiplier at the end. So you just need to score 9000 before the finish. I was able to get just above the 9000 mark by killing a few of the enemies in the final area where you run to the longsword ship. Make sure you kill all hunters with pistol as they are worth ~500 each. You can take out most groups of enemies with plasma grenades. Boom skull turns them into mini-nukes. You will have near unlimited stock as you can melee the flood carriers for plasma grenades with the pinata skull on.
I had tried this before on Legendary with no scoring skulls enabled and didn't get it because it took me too long.