Infinite/Astral Zamasu solos hard. Seiya has no way affecting an omnipresent astral being which took Zeno to wipe out entire timeline housing multiple Universes just to erase him and his immortality out of existence.
Since he also has the 8th Sense and therefore, full control over his own soul, he's able to interact with spiritual beings like Hades' Specters or even Hades himself in astral form. For the record, spiritual attacks are pretty common in the Seiya verse. Even fodder Bronze Knights have felled Hades' Specters with a single swoop: