The AC having a few means of reducing it, is intentional. This is the theme of the game: Erebus is a dangerous place at the very start of the game, and as each turn passes, the world becomes even more broken and chaotic, literally the "going to hell" expression. It also does serve an important game mechanic, in keeping the game more interesting (and challenging), as you progress longer into the game. Without it, continue playing would be boring, as you'd be dominating the world, with nothing to do in the game. It allows for you to enjoy the late game technologies, spells, units, strategies, tactics, and etc, as you try to just survive against the rising AC's challenges. This is a loose paraphrasing of a post by Kael somewhere, talking about this very design decision of having so few means of stopping the AC from rising.
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The only way to keep it from rising, is being (or through) the Elohim civilization, as they have a means (a ritual, I think?) of reducing the AC unlimitedly (...I think).
This is why the Elohim is a very good civilization for new players, along with being a mostly normal (Civ4/BTS) civilization, compared to the more exotic civilizations like the Khazad (vault mechanics feature unique to FFH2, not in Civ4/BTS), Kuriotates (limited quantity but super/3rd ring cities + settlement "cities" limited just for resources), Illians (tundra terraforming and etc features), any of the magic focused civs, and etc exotic civs.
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This is very very very old, but I think it still gives a good synapsis of the civs, FFH2 features, and etc for new players:
http://forums.civfanatics.com/showthread.php?t=308987
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