Warhammer 40,000: Shootas, Blood & Teef

Like the Space Marine kills, the best level for this is Castellum Luteus on Easy. You run into Tempestus Scions (the blue armoured Imperial Guard) throughout the level in decent numbers. Once you reach the White Scars Chaplain, it's best to restart the level and continue killing them until you have the achievement.

Video below if you need a visual reminder of the level (credit to Dreemax for the video).