Rimefire Plate would reduce Rimehound Bite damage by 2, or 1 for each type.
RAW:
Resistance: Resistance means a creature takes less damage from a specific damage type.
Resistance, Against Combined Damage Types: A creature's resistance is ineffective against combined damage types unless the creature has resistance to each of the damage types, and then only the weakest of the resistances applies.
Example: A creature has resist 10 lightning and resist 5 thunder, and an attack deals 15 lightning and thunder damage to it. The creature takes 10 lightning and thunder damage.
Resistance, Not Cumulative: Resistances against the same damage type
are not cumulative. Only the highest resistance applies. Example: If a
creature has resist 5 cold and then gains resist 10 cold, it now has
resist 10 cold, not resist 15 cold.
Damage Type: Powers and other effects often deal specific types of damage. ... Most weapon attack powers deal damage that has no type. It is simply physical damage.
The keyword here is "damage type". Resistant does not stack against same damage type. However, for consistencycombined damage type is different from multiple damage type, as written:
Fell Court Hellmage, Infernal Bolt (MV:TNV p.51)
Hit: 1d8 + 3 fire and radiant damage.
verse:
Sunsearer, Sunstrike (The Plane Below, p.139)
1d6+3 fire damage plus 1d6+3 radiant damage.
The rule on combined damage type specify what happens to X fire and radiant damage
, but does not specify what happens about X fire damage and/plus X radiant damage
.
So what happens? We fall back to the general rule: Resistance means a creature takes less damage from a specific damage type.
The rule on not cumulative resistances does not applies to damage roll, as written.
And we do have two damage types in case of sunsearer (in one damage roll).
If you have fire or cold resistance, you independently apply your resistant to each damage type.
Your fire resistance does not help you resist cold damage, even when it is one same damage roll, and vice versa.
If you have fire and cold resistance, they are still applied independently. If you have fire and cold resistance from the same source, they are still two different resistances, and are still applied independently.
The new resistance rules does not change this mechanic. It just clarifies how you apply resistance to combined damage type, but only when you run into them. There exists no rules that make resist all apply differently from specific resistances.
RAI:
While applying resistance once for each damage roll seems to be a easy way out, it collapses when facing anything more than a simple resist all.
RAW and RAI, cold damage
is different from fire damage
, and both are different from cold and fire damage
.
Resistance doesn't stack, but only for the same damage type.