How does elemental equilibrium when you deal ice/fire in a single attack?

You hit with fire and cold, so the monster hit gets +25% resistance to those two elements. Doesn't matter where they came from, only that you dealt those damage types in that hit.
You did not hit with lightning, so it gets -50% resistance to that.

EDIT: Malice is correct as well

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BRavich wrote:

How long does the effect last? I am guessing that it is until the next attack inflicted on the enemy but I cant be sure.

Either 5 seconds or until the next time EE would affect that enemy, whichever happens first