My brother finally decided to listen to the voice of reason and we finally started winning! We played 3 games:
(1)
Ancient One: Y'Golonac
Modifier: Nodens
Invesitgators: 5
Outcome: Win by sealing 6 gates
(2)
Ancient One: Yog-Sothoth
Modifier: Hypnos
Invesitgators: 5
Outcome: Win by sealing 6 gates and having a close-all-gates victory at the same time
(3)
Ancient One: Hastur
Modifier: Bast
Invesitgators: 5
Outcome: Win by salying Hastur in the final battle (epic battle variant)
I think we're finally ready to play without guardians. And then, maybe, we shall be ready for heralds.