Fireball
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1. True Luck
2-3. Scream of Terror (especially with a Dread Knight) and Call of Urgash.
4. Fury of the Elements (it's just preferable not to have only one native castle; this is a slight drawback).
5-6. Knight and Mage - forget about it.
The most important thing is that it's significantly easier and with fewer developmental losses to obtain the Ultimate skill for the first three. Only the Elf can do this with just 4 heroes, while the Necropolis, for example, can do it with everyone except Zoltan.
2-3. Scream of Terror (especially with a Dread Knight) and Call of Urgash.
4. Fury of the Elements (it's just preferable not to have only one native castle; this is a slight drawback).
5-6. Knight and Mage - forget about it.
The most important thing is that it's significantly easier and with fewer developmental losses to obtain the Ultimate skill for the first three. Only the Elf can do this with just 4 heroes, while the Necropolis, for example, can do it with everyone except Zoltan.
