Last time I check, Jiren's not Omnipresent, neither Omnicient.
Again, Shields, reducing the damage in half, and tanking hit from someone that also superior to him, like say Giygas, should give him the durabilitie to tank those hit with Shields, or either whitouth them, since he doesn't One Shot either way. And Ness can go full heal in an Instant.
After Magican is where they go to fight Giygas, wich is where he uses all the power he gains to fight him in the firts place, Omnipresence and Omnicience. Thats the whole point of Magican.
Even then, going by the standar battle assumptions, both character are at the peak of power, wich means when both characters had all the power they add, so, taking in count how Ness's Low-2C power is in every form after Magican, and that he uses this power to specifically fight Giygas in his robot form, he should have by default Immortality type 6, since that where he was at his fullest.