The description is a little unclear, but the actual achievement is straightforward. The “Raider” is the Space Marines’ Land Raider tank, a ninth-tier armored unit. Damage a Land Raider below 50% health, and then finish it with a Chaos Lord’s Exalted Strike ability. Land Raiders have a lot of health and armor, so you may want to buy a few weapons at the Trader Encampment for the Lord. Also, he’ll need to be level 6 to use Exalted Strike. [In retrospect, I didn’t try to get the achievement *without* Exalted Strike, so any killing blow may unlock the badge. Now confirmed, you do need to use Exalted Strike to complete the achievement (see the Discussion).]
If you want to ensure that you encounter a Land Raider, you can play through the Chaos campaign storyline. The final encounter will spawn waves of Space Marine vehicles, including Land Raiders.
Notes: Ram raiding is the act of driving a heavy vehicle through a storefront, typically in order to rob it. A “Raider” is also a blockish Dodge Ram 4x4 sports utility vehicle from the 1980s. While neither really fits the achievement, I assume the reference is to the first phrase…?