To solo UT a char dishing out lots of physical or magical damage is best suited. Go melee paladin and barb, hammerdin, kicksin or summon necro.
Magic damage absolutely is the not the way to go. Meph and Baal both have 75% magic resist. How do you expect to take advantage of lifetap if you using magic damage? Hammerdin will need to switch weapon and use smite.
Q u o t e: Very strong lightning damage dealers like javazons and light sorcs do too but they need to have either good Lower Resist curse or a Conviction aura around.
There is no way any merc can cancel meph's conviction. And no lower resist wand can go below -41% resist which is not enough to break anyone's immunity.