[AdmiralZhao]

In general, magic is used to create combos that national troops cannot handle and to counter combos that your national troops cannot handle. In your question you describe the first case, where at research level 7 you start to see battle winning spells (Mass Protection, Fog Warriors, Darkness, etc.) that let you massacre unsupported national troops. And so long as your opponents just use human-level troops against you, you can get away with not using magic, or in waiting until you've invested enough use the high level spells.
However, if you are facing the second case where your opponent has something your national troops cannot handle (elephants, combat pretender, blessed sacreds, etc), magic is needed to counter their advantage. You see this primarily in MP, since in SP the AI isn't smart enough to put together vicious combos.


For the particular case you gave with EA C'tis, I'd recommend that old stand by of skelespam. While your casters are initially expensive, they do not take any casualties so long as they win the battle. With a normal army, you lose a portion of your troops after each battle, but the casters can keep fighting indefinitely without losses. And once you reach Evo 7, you can combine the skeletons with Cloud of Death, and then things become really fun.