Yuri_G
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...The "Summon Phantom" spell should be chosen when selecting a bonus in the 2nd mission. It remains for all subsequent missions. This spell instantly creates a clone of the selected unit. However, this clone is weak – it dies with the slightest damage. But if you don't let dragons get close to it, the phantom works great for demonesses. Just make sure there is space next to the unit being cloned for the clone...
Why is the clone weak? In my case, phantoms died every other time, i.e., they could withstand one hit, but died on the second. In most cases, that's exactly what happened.