Generally resist reduction that can break immunities will work with 20% effectiveness: this is for conviction aura and lower resist curse as far as I know.
If you have a sunder charm, that will automatically put the immune mobs to 95% resist, from here onwards:
- Conviction will reduce it further with 20% effectiveness (i.e. Infinity has lvl 12 aura, this would be -85% against non-immunes, -17% against immunes)
- Lower resist will reduce it further with 20% effectiveness (i.e. wand with lvl 3 lower resist charges, this would be -41% against non-immunes, -8% against immunes)
- Cold mastery cannot break immunities, but it will work only with 20% effectiveness on originally immune mobs. This was patched in D2R shortly after the original release of sunder charms. If I remember correctly cold sorc was simply broken, because everything had -100% cold resist with a sunder + cold mastery
- Any -resist gear you might have (the tal orb you mentioned, crescent moon, rainbow facets, etc.) will work with 100% effectiveness, but they will not break immunities on their own. Note that the mercenary's Infinity could have e.g. -50 light resist, but that will only benefit the mercenary's lightning damage, not yours. However if you equipped the Infinity yourself, it would work.