After three days, was finally able to beat Eirin and Kaguya in the Genius of Sappheiros. In order to do that I recruited "secret" character in my party, Byakuren Hijiri, leveled up her from level 1 to level 20, completely changed equipment, skills, abilities of several characters, party members and party formation and ultimately I did it.
The problem with those two bosses, is that Kaguya buffs Eirin's attack and defense and Eirin heals Kaguya and herself for several thousand HP during battle. Even when I was able to defeat Eirin, I didn't have enough MP to use any spells on Kaguya after that, and she slowly killed me. To defeat them, I used Byakuren spell "Control", that allows to take control over enemy character, and used Kaguya's buffs for my party. At the same time I bombarded both bosses with status debuffs of all possible kind. Eventually, that was enough to keep Kaguya under control for 6-7 turns and Eirin didn't try to heal herself for 2000-3000 HP like she usually does. After that I had enough MP to keep Kaguya under debuffs and destroy her.
If bosses of the level 10 were that hard, how hard will be bosses of the next levels...?