Uber Mephisto's definitely has level 20 Conviction, applying Fire, Lightning and Cold Resist -125%. This applies to resistance values before any ceiling: For example, resist 180% exactly is reduced to 55%, or -45% after the Resist -100% penalty in Hell; Resist 200% is reduced to 75%, or only -25% after the penalty; Resist 350% is reduced to 225%, or 125% after the penalty, then the ceiling is applied.
In the absence of Conviction, the ceiling would normally result in Resist 75% being displayed (80% with +5% Maximum Resist) in all those cases.
You can test it yourself with Resist 26-75% after the Hell penalty, which should result in Resist -(50-99)% being displayed. The only target within radius to which it does not apply is a Paladin already supplying level > 20 Conviction himself.
Taken from a D2wikia source.
He has
too high of lightning immunity and his conviction is higher than infinity's so it would be overridden.
You won't be able to break his immunity.
http://wiki.theamazonbasin.com/index.php/ConvictionQuote
Stat Value
Radius (Yards) 13 1/3
-% Defense min(40 + [60 * [ (110*slvl) / (slvl+6) ] / 100] , 100)
Fire Resist -% min(25 + (5*slvl) , 150)
Lightning Resist -% min(25 + (5*slvl) , 150)
Cold Resist -% min(25 + (5*slvl) , 150)
You'll need to get about 350% as shown above.