I'm 100% sure it's -125% res, i.e. L20 and I'll take any bet on it.Uber Mephisto has 25 lvl Conviction aura whith do -150 all res.
So you need at least 26 lvl Conviction to cancel it.
The level of conviction as a random boss ability is mlvl/8, but Ãœber Meph is special anyway.It's normally MLvl/4, and Arreat Summit states 110, making it 22. Are uber uniques specifically set, or is that the same with any super unique?