Say I have a Javazon with a lightning sunder charm, griffon’s eye, merc with infinity, and a lower resist wand for the hell of it.
Which one of these has top priority in breaking the immunity? Sunder charm? Once the immunity is broken do the other sources of -lightning resist all work fully or all they all reduced to a 1/5th of their power?
yes -resist gear/facets work best with a sunder charm, infinity and lower resist have reduced effects on monster immunity in conjunction with sunder charm.
Cold mastery also has reduced effects, you would need to find the patch notes to verify this for yourself.
Sunder breaks the immunity as first priority (setting it to 95%), then other sources of -res lower further from there. -res from gear works at full strength, but -res from Lower Resist, Conviction, and Cold Mastery only work at 1/5th their normal value on formerly immune enemies.