I got it by making a custom arena. Whenever you do that you can select which terrain you want to use. All of the available terrains for AH are there.
Go online and from the main arena screen click on the player tab. From there click create arena. A dialog box will appear. Name your arena and select a terrain from the list. You can leave the other stuff alone. You're just grabbing terrains.
Click create. The chosen terrain will download and your arena will open.
To close the arena: clipboard/options/CM tools/shutdown arena. It'll kick you as if you lost host or the war ended and your arena is gone. You now have that terrain in your terrains folder.
Hope that gets what you were looking for.
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