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1) Warriors are almost always better.
The Shield Wall is a very useful ability. Especially when capturing mines and against neutral creatures in general. And if the hero is Ingvar, they become not just cannon fodder, but the best tier 1 unit, which can easily compete with tier 2 units of other factions.
2) Spear Masters are almost always better.
Crippling Strike, which pushes back on the initiative scale, is always better than Harpoon.
3) They are more or less equivalent. But I vote for the Northern Riders.
The Claw Attack is a less predictable ability and more difficult to use. The Frightful Howl looks better and the opponent is guaranteed not to retaliate.
4) They are equivalent. I like the Arcata Warriors more.
In general, I build Berserkers at the very end and only if all the others are bought out. Otherwise, they just sit in the castle. That's why I prefer the more robust Arcata Warriors. Because the Berserker's ability is quite difficult to apply against an opponent, as there may always be our own units nearby, and the Berserker attacks a random adjacent unit.
5) Against neutral creatures, it is definitely better to take Flame Priests. Fireball (especially with Light Magic) is more useful.
But against enemy heroes, there are many options to choose from. I like the Arcata Priests more because of the cross attack, which can almost always hit two creatures.
But you can also make good combinations with Flame Priests. After all, they and the Lords can cast the Fire Brand, and in combination with Fireball, this can be more advantageous than the cross attack.
6) I'm always in doubt. Both are very good.
If there are Flame Priests, then Lords are better - thanks to the Fire Brand, you can reliably get rid of one stack of the enemy, thanks to the Fireballs.
But I like Erls more. After all, with Berserk Runes and War Cry, they are often more useful. It's especially nice when the opponent stands shoulder to shoulder. You hit the weakest one, and all of them receive the same damage, and the retaliation is the least damaging to us.
7) Lava Dragons are much better. Dragon's Breath is better than a small Fire Wall in 99% of cases. And they are also better protected, especially if the hero has a defense skill.