Mana siphoner and lightning dot vs Doryani's Prototype

You have a mistake in the OP because this damage isn't considering your lightning resistance at all. Having -200 does not increase the damage taken over having 0%. You are counting as if you have 0% resistance. Which is still pretty bad because elemental damage is balanced towards having 75% resistance - you are taking 4 times as much as intended which is enough to kill you very quickly.

This means that Elemental Weakness or Conductivity curse has no effect (towards any lightning damage) either, as those curses lower your resistances.

There is a way to bypass this weakness by converting entire lightning damage to other elements via Lethal Pride (tempered by war) and the shield from Trialmaster. Although this is quite clunky because this shield has 6 keystone skills on it. And they don't work well on CI builds.