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If you're playing as a mage and have a large map, you can easily max out all the magical skills (intelligence, wisdom, magic, mysticism) and 4 elemental magic schools. But generally, it's better to upgrade them one at a time, rather than all together, because having 4 elemental schools at a basic level is less useful than having 1 expert-level and 1 basic-level school. First, I try to max out air magic – the mass haste spell allows almost the entire army to reach the enemy on the very first turn, and I prefer chain lightning to explosion because, with a well-developed power, the explosion is often too strong, while using chain lightning to eliminate 5 units is quite effective! If the choice is between earth or water/fire, I won't even think about it. And not because of the explosion, but because of the mass slow effect. It's so satisfying to torment an opponent whose units are five times slower than yours!
I remember how disappointed I was when I first discovered that many spells at the expert level affect everyone, and I started upgrading fire magic for mass blinding...
I almost never choose water magic, except maybe for the sake of completing the collection when only one slot is left.
When playing as a warrior, I either upgrade one specific school or air and earth, but I prefer to upgrade logistics and pathfinding rather than rely on the chance of finding a town portal in one of the cities.