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Sometimes, you don't need much; one or two spells are enough to win. It's pointless to use Earth Shock on, say, ten Imps; Lightning will suffice. It's like using a cannon to shoot at sparrows – you'll kill them, but you'll waste mana. It's better to conserve mana, as that's often very important. Of course, at a high level, you can't upgrade much, but you can buy everything you need; the main thing is to have enough money.