Okay.
Going by his page, Seiya has:
"
Resistance to Mind Manipulation, Soul Manipulation, Matter Manipulation, Illusions, Dream Manipulation, Life Manipulation and Death Manipulation, Space-Time Manipulation, Existence Erasure." Let's explain this in detail.
Mind Manipulation.
Well, a lot of this has to do with
Virgo Shaka. Shaka can casually erase your memories and also restore them, he can trap you in illusions so life-like and powerful they are called "real illusions", and affect you physically. His
Buddha's Hand technique is ridiculously powerful and designed to instil fear on the opponent, who believes Shaka to be a god and flees in terror, believing to be crossing the entire universe to flee him, only to discover he is trapped under Shaka's reach like a monkey on buddha's hand. Shaka can also erase your 5 senses and also destroy your mind.
Phoenix Ikki overcomes all of that, either easily or with difficulty. With the 7th sense, you can literally have your mind destroyed and your 7th sense will replace the mind for you with no consequences. Oh, and a fodder Bronze Saint had his nervous system literally destroyed by base Ikki, and while he went to the hospital he eventually got fine.
Soul Manipulation
Everyone and their mother can affect souls and even destroy them. Seiya tanked many attacks from beings who can throughout the series. Cancer Deathmask can rip your soul and cast it into hell, and less powerful characters than God Seiya can survive and reverse that.
Matter Manipulation
Everyone and their mother in Saint Seiya destroy atoms and can punch each other. As shown in the Poseidon Arc, where the Saints can't destroy Poseidon's 7 Pillars because they are protected by his Cosmos, needing the Libra Weapons to destroy them, which later prove innefective against the Mainstain Pillar that made from Poseidon's cosmos, the more powerful you are in Saint Seiya the more difficult you are to be affected by Matter Manipulation. Of course, comparable and stronger characters could manipulate Seiya's matter, but weaker? Out of the question.
Illusions
The "Buddha's Hand" technique I mentioned earlier? Gemini Saga casually destroyed it with his cosmos. So Seiya's great cosmos could nullify illusions from weaker characters easily.
Dream Manipulation
Hypnos can put you in a state of eternal sleep and endless dreaming. The God Saints get hit from it, and after being put in eternal sleep, they break from it seconds later through sheer cosmos.
Life & Death Manipulation
Thanatos controls death throughout the whole universe. He can kill you and kill soul with nothing but a thought. He uses his strongest attack against Seiya at full power, and it doesn't do shit.
Space-Time Manipulation
Seiya survived in the Hyperdimension, which distorts space-time and erases all who aren't gods from existence.
Existence Erasure
See above. Also Seiya suffers from Hades' Curse. Basically, the wounds caused by Hades' Sword are eternal. Once hit by it, the wound shall remain forever. Even if you heal physically, it will remain affecting your cosmos and soul. Seiya had
his heart pierced by Hades' sword, and yet he lives. He was left in a catatonic state for years, but eventually got better (See Episode G: Assassin). The explanation is that his cosmos and will is so great that he put himself in a state of constant miracle, perpetually blocking the wound from killing him.