Ambassador

The first dwarf is still manageable. But the second one should, in theory, attack after you capture the city. Well, after that, everything becomes easy. Nikolay outlined a very effective tactic. And the hero's special ability helps – two, or even three, enemy units are guaranteed to miss two or three turns if you just shoot at them. There's also the Lion's artifact on the map.
The battles with the outpost and the necromancer guard aren't the most difficult – the enemy doesn't have a hero. Moreover, the faster you break through the outpost itself, the easier it will be overall. However, there seems to be some kind of bug with the necromancers – or rather, not a bug: they somehow noticed and actively destroyed the crystals of secrecy, even though I placed them correctly; I don't know why. But you can also win that battle with brute force – which is what I ended up doing.
The last battle on the map is relatively easy – fighting a weak human with your upgraded human isn't that hard.
I think the most difficult battles are the fight with the Guardian before the nerf – without Darkness, it's impossible; he has huge resistance. And yes, the fight with Helmar requires you to try and win without losses.