Theoretically, a HS Cain, with a NP based on God's promise to strike sevenfold whoever harms Cain, coupled with the recognition because of the advent of Christianity in the modern world, might mean he can draw even with Gil at his strongest.
But that duel means mutually assured destruction for Earth itself as it means Ea being reflected back sevenfold.
Anyone, shouldn't Cain have some level of MEoDP? I mean, as legend has it, he invented murder. Going by what we know of Aviceborn's NP, ADAM was on his way to total immortality because there was no death in Eden. Cain brought death to Eden. By Nasu logic, he can conceptualize lines and dots of death on things that by the very law of nature, shouldn't have them.
Gonna try brainstorming some other legendary figure on that level. If there are any. Gotta be at least as ancient as Gil anyway, cause it needs a broken OP legend.