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The very first time, I chose the Warrior class. In my opinion, it's easiest to beat the Dark Messiah with a Warrior. But I wouldn't say it's *much* easier. Sometimes there are difficulties with the Orcs, but they can be defeated with various map tactics.
The second time, I played as a Mage. And you know what? I really, really liked the Mage class :) Not only is it just cool, but it's also fun! Oh, how I laughed when I froze a sharp turn (90 degrees) on a cliff. The Orcs run after me, slip, and fall off the cliff, hitting their heads hard on the boards they slipped on))) Or shrinking enemies down to the size of a cat! My younger brother (6 years old) was lucky enough to see me use that spell. There was so much laughter, both from me and from him!
I started playing as an Assassin a couple of times, but I never finished. Combining specializations doesn't always bring the desired result. It's better to level up a pure Warrior/Mage/Assassin; it's simpler and better. In general, for a complete picture of the game, it's best to play through it 3 times with all 3 specializations. Personally, I got the most enjoyment from the Mage hero.