Well, my thought process was basically, Ea can destroy the world no problem, the Grail can create Ea, so assuming the weakest possible Grail that still fits with canon, Odyssial's minimum offensive power level is "can destroy the world no problem."
Avalon is a nigh-perfect defense against pretty much anything in the setting, so assuming the weakest possible Grail that still fits with canon, Odyssial's minimum defensive power level is "nigh-perfect defense against pretty much anything in the setting."
Different configurations of matter / energy / prana can yield different levels of actual power; consider a uranium deposit vs a nuclear bomb, or a normal person vs Aoko Aozaki. The difference in mass-energy is not high, but the effective potential is astronomically variable. Humans could depopulate Gaia fairly easily, so it's extremely likely that the existence of a prana budget for Odyssial will not yield an Odyssial so weak he would have the remotest trouble with the setting. It's simply not fair; it's like the Grail summoning a Xeelee self-replicator, only more of a stomp than that, because at least the Xelee are affected by time travel. This is like dropping Mystra into Westeros.
We know it is possible for exceptionally talented humans to attain a Magic.
Assuming High First Age / Pre-Usurpation Odyssial, he will within femtoseconds of arrival deduce all Five Magics*, attain infinite energy via the Kaleidoscope / Aoi Magic, immediately restore himself to actual full power, and then stomp. If somehow he cannot attain the Magics, he will spontaneously generate a Reality Marble with the attributes necessary to restore himself to full power and contain it within himself Nrvnqsr-style, and then stomp. This is assuming that the strength of his Reality Marble does not immediately overwrite Gaia / ORT given his much higher conceptual / paradigmatic priority.
Remember, Odyssial is stronger than the Unconquered Sun, so any Odyssial produced by the Grail's energy budget would be at least stronger than any UCS produced by the Grail's energy budget. He is so ludicrously overpowered that he shouldn't really be used in any crossover except maybe with cosmic-level soft science fiction.
*Thank god that 3E removed ticks so we're not locked into seconds being a relevant unit of time for E10 combat.