In principle, creating battle arenas isn't that difficult. Everything is done through the map editor (at least, that's how I do it). If you're interested, I can write a sort of 'homemade' guide.
By the way, I still can't figure out how to take screenshots in the editor. I know there's a MakeScreenshot button, but it only captures the working area, cutting off the window on the left (MapPropertiesTree), and usually, that's the window you need to screenshot.
I figured it out after looking at what Dyrman posted above, so thanks are no longer needed.
Multiplayer: Господство, Извечное противостояние, Война за Магию, Торговая империя, Анатомия войны, Эксперимент
SingleScenario: Последний Рубеж, Трон, Элиралис, Вернуться домой!, Лабиринты разума
Campaign: Колонизация