If you want to try and get this by yourself against the AI, it can be difficult, but it's still humanly possible. Here's what I did:
Play against another Ironheade (Most Gentle AI, of course) on Feeding Area. It could probably work on other maps, but I used Feeding Area. Cap all the geysers except what the AI started with and defend them until you can upgrade your stage all the way and build a Rock Crusher.
Double team with it and drive it over to the AI stage area. If he's built up an army there, use the Bladehenge Assault to make them not alive anymore. When he summons more troops (likely Headbangers), drive in and start running them over. It should take 2 or 3 crushings to kill them. If they start ganging up on you, drive down a road leading away from the stage and get some distance so you can turn around and point the business end of your Rock Crusher at them.
The AI avatar will be your biggest problem. I don't know if avatar kills count towards the achievement, but it's a good idea to keep running him over anyway to keep him out of your hair for a few seconds. If you have subtitles turned on, look for one of his death quotes so you know when to turn your focus towards the troops.
With this method, I got the achievement before my Rock Crusher had even taken enough damage to catch fire.