Magefists offers increased Mana regeneration, which should speed up, ever so slightly your mana regen.

What is unfortunate about -enemy resistance is that the hit only applies to the mercenary. 55% would help, but once you have 50k Lightning, that's not much of a problem.

Lifesteal shouldn't be too much a problem, especially because with a pure Lightning Sorceress, everything except Ubers will be dead by the time they get onscreen. If you've got a good Smiter helping you out, everything will be dead.

As for a character revamp, it shouldn't be too hard to find what you need, seeing as we are all one big family here, we should have, if not optimal, sufficient equipment.

Once immunity is broekn, then the remaning minus resistance works at 1/5 capacitiy. For example, if I had -150% Lightning resistance, and an enemy has 130% Lightning resistance, it would chop off 30 to bring it below 100%, then the remaning -120% would be divided by five, 24%, and subtracted from 100%, leaving your total resistance at 76%.

But once your damage exceeds 10k (which is possible even with base skills), that's 25% of 4 to ~10000, 1 to ~2500 per shot, at 2 shots per second, for 4 seconds on a typical enemy. An average comes out to 1250*2*4. An impressive amount, even for Final Fantasy standards, but as we know, Diablo characters would own Final Fantasy, even on their own system. Frenzy wins Sephiroth. It's been proven.