Heart of the Night

It's unclear whether the mass phantom and simultaneous summoning are a new spell or just a random bug (version 3.1). And regarding the ease of the battle – is that a joke? The first seven shots in a row (before my units even moved) decimated all the archers and weaker units, making it pointless to continue.

Added 6 minutes later
I can't open the attachment; could you please upload the save file without archiving it?

As I mentioned before, the map is designed for the player to strictly fulfill certain conditions. Any deviation leads to a swift defeat:smile31:.
In the case of Orris, the mandatory conditions are:
1) Having the "Dragon's Eye" ring (hidden near the castle, guarded by demonic mares). Then, the blue archers shoot first and eliminate the skeleton archers.
2) Greta must have the "Arrow Storm" skill, preferably with an enchanted arrow. It's also essential to re-select sworn enemies for each Necropolis hero individually. For Orris, these are bone archers, liches, and vampires. If you charge Arrow Storm with a slowing effect, in addition to guaranteeing the destruction of phantoms, it also slows down the vampires.
It's desirable to:
1) Develop Greta – having a mass evasion skill that can be cast at the beginning significantly eases the battle. Therefore, an expert light creature with protection and storm wind (against vampires) is needed. It's clear that to have this "at the beginning," you need a leader with empathy, so that morale-boosting fairies can move Greta up the initiative scale.
2) Be sure to visit all the upgrade locations on the correct days.
3) Prioritize eliminating all the archers belonging to Orris; if you're lucky, you can manage to phantom your own archers. At the same time, you also need to eliminate the elementals as a priority, as they don't 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. Otherwise, you get what you experienced – with no options.
You can watch the battle at Miru-mir's by creating a "Replays" folder in your game profile and uploading it through the "battle recording" option in the settings.