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Yeah, the 5th one is the toughest! I played it like this: Raela, Chaos Center, Peru, and generally a standard build, with damage dealers around 7k with elemental damage and BDP under Invisibility. Ilaya is the most difficult at the beginning; I leveled her up with Light in the beginning, which is the hardest for her, but on the surface, it's very easy. Bill was a combat unit, Solitary, Vengeance, Light, and Doga (not sure), and something else.
Gad is about summoning under Invisibility, also BDP, and in the beginning, fire elementals carried, then Phoenix became a real powerhouse. Generally, Charisma, Education, BDP, Summoning, and Horse, but I'm not sure about the last one. It's easy in the dungeon, but long and tedious on the surface.
Tralisai is Darkness, Charisma, Education, and the rest as desired.
Tralisai and Ilaya had armies. Tralisai had red dragons under Vampirism, there were about 40-50 of them, and he also had defense. I don't remember Ilaya's army; if I remember, I'll write it down. I advise clearing the dungeons with Sigrif because there are upgrades in them that he has, and it's impossible to pass the last two battles without Darkness, although you can somehow manage with a Phoenix with 150 Colossus and cast Heavenly Shield in the castle, but the last battle is impossible without Darkness.