Rimefire Plate would reduce Rimehound Bite damage by 2, or 1 for each type.
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, combined 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.
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.
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.
Let's say an attack deals
10 fire damage and 8 cold damage.
If the target has both
resist 6 fire and
resist 5 cold, you have to decide what to do.
If you say the higher applies, what happens when the damage is
1 fire damage and 17 cold damage?
- Do you stand by resist 6 fire and say 5 point of fire resistance bleeds out into the cold damage?
- Do you say at most 1 fire resistance apply, so that cold resist 5 is now higher?
If you say the lowest applies, again what happens when the damage is
17 fire damage and 1 cold damage?
- Do you stand by resist 5 cold, confusing it with
18 fire and cold damage?
- Do you say at most 1 cold resistance apply? It is easier to give up and calculate them independently.
If you say both resistances applies, what happens when you have
10 fire damage + 8 cold damage + 2 fire and cold damage?
You have to take high or take low for both resistances, which leads to same problems.
If you are still not convinced, many ghost like creatures has
Resist insubstantial; Vulnerable 5 psychic.
How are you going to apply insubstantial resistance when it is dealt untyped and psychic damage in one blow?
What if another creature gave it some mixed resistance? (It happened in my game)
Additionally, some answers may indirectly encourage using whetstone to bypass resistance, or gaining resistance may increase received damage, or make it difficult to decide what type of damage did the target receive (some effect is only triggered by specific damage type).
By calculating immunities, resistances, and vulnerabilities independently for each damage type, you keep you sanity while getting believable result.