In Hell difficulty, monsters can have resistances over 100% — making them immune to that damage type. Immune monsters cannot be killed by that element without a Conviction Aura or Lower Resist curse to break through. This makes your farming area selection crucial.

Green — No monsters in this area are immune to any of your selected damage types. Farm freely.

Yellow — Some monsters are immune to one or more of your damage types, but not all. You can still kill most things here.

Red — Monsters in this area are immune to every damage type you selected. You won't be able to kill them.

Blizzard Sorc: Ancient Tunnels (no cold immunes, alvl85), Mausoleum, Arachnid Lair.

Hammerdin: Nearly everywhere — Magic immunes are extremely rare. The Pit, Chaos Sanctuary, Worldstone Keep.

Lightning Javazon: Ancient Tunnels, The Pit (watch for lightning immunes in some areas).

Fire Sorc: More restrictive. Look for areas with few or no fire immunes — use the calculator above.

Sunder Grand Charms (added in Patch 2.5) break monster immunities for a specific damage type by setting that resistance to 95% — not removing it. Resistance stays very high, so you still need stacked -enemy resist (Conviction, Lower Resist, Cold Mastery, facets) to deal meaningful damage. There is a Sunder Charm for each of Fire, Cold, Lightning, Poison, Physical, and Magic.