Heart of the Night

In the case of Orris, the mandatory conditions are:
1) Having the "Dragon Eye" ring. Then, the blue archers shoot first and eliminate the skeleton archer stack.
2) Greta having the "Arrow Shower" skill, preferably with an enchanted arrow.
Desirable:
1) Greta's development - having a mass evasion skill that can be cast at the beginning.
2) Visiting all the upgrade locations on the correct days.
3) Prioritizing the elimination of all archers from Orris, and if successful, phasing your own archers. At the same time, elementals should also be eliminated first, as they do not disappear when the rest of the army is destroyed.
4) An unpleasant surprise for Orris is Greta having "Elemental Equilibrium" - water elementals appear, which are effective against phantoms.
If these conditions are met, as Miru-mir wrote, everything is easy.

Here is my Greta before the battle with Orris.
Point by point:
1) The Dragon Eye increases the initiative of the archers - it's a must-have, and it works great with the Dragon Crown. Ideally, Karlam should acquire the ring (which requires some effort) and give it to her. This is the easiest way. But again, this is a matter of creativity - you need to think about how to develop Karlam so that the start is as easy as possible for him. I gave all the artifacts to Greta, except for the clover, and under no circumstances should you sell the Armor of Fearlessness! It's very useful, even though it's cheap. I had to replay a large part of the map because of that.
2) I didn't use Arrow Shower at all, nor did I use the enchanted arrow. The priority is to increase initiative, i.e., I would rather spend a turn on mass acceleration, and maximize Dragon Eye, and then the accelerated units will demoralize faster, plus the phantom effect. And Greta sympathizes with them. In general, while the poor necromancer is still doing something, we can already destroy a third of his army.

1) Greta. Storm Wind is definitely essential everywhere - it's effective against dragons, vampires, and ghosts. Naturally, Wrath - you must accelerate in every battle. And Blessing - this is mass spell removal and Divine Might on the archers = unicorns, children of light. That's it, 3 perks.
I didn't take protection (mass evasion), including in the battle with Orris, because if we increase the initiative, then by the time his archers (sworn enemies) take their turn, not much will be left. Well, he might shoot half a stack of liches at the fairies (we'll resurrect them), and the phantom will shoot at our phantom. There are no problems at all. The fire elementals might shoot once or twice.
And while the ghosts and vampires are slowly approaching under the storm, we will already have eliminated them.
2) Yes, in the "random" Sanctuary, initiative is available on Fridays.
3) Yes, the archers are sworn enemies, we eliminate the liches, maybe not all of them, and the skeletons, and that's it.
4) I didn't take Elemental Equilibrium. I took Misty Veil - this will also slow down the liches and skeletons at the beginning.

Regarding Karlam, there are also options for creativity. For a quick start, I took Machines and Chaos. We need ice to slow down enemies, ballista for damage, tent and regeneration - to save troops. Education and Coexistence are tempting, of course, but I took Enchantment and Attack (shooting and ballista). And that's how I passed through to Turgal without any problems. In my opinion, without Machines, we won't be able to destroy the horses to capture the ring and give it to Greta. And she needs it so that she can quickly eliminate everyone.
His journey is, of course, standard, but there is also room for thought to avoid being caught by the demon and to plan the route.