Elemental to whatever damage conversion. And a second question regarding Aeon's.

[Superfluff]

Example:
NB’s deal primarily cold damage (in terms of elemental damage). If I have 100% elemental to pierce conversion, would I only convert 33% of my cold damage? Or would I convert all of it?

nono, all of it

Where it get’s tricky is for example cold>phys vs elemental> phys.

Let’s say you have 50% cold to phys and 30% elemental to phys. They are not additive. meaning you won’t get 80% total. One comes first ( not sure which, probably the elemental one) then comes the other to convert what is left, so it;s multiplicative .

same goes for all other global vs focused conversion. Like Cord of violent decay and decree of aldritch. You won’t get 100% fire to aether even with good rolls.