I now know what "P" resist is

How "P" resist works:
1. Any skill that does not have elemAttr defined on it is affected (Lucky 7, Power Strike, Brandish, Magic Claw). Skills that have elemAttr defined do regular damage.
2. Regular attacks are not affected (gun whack, bow whack, single star throw or punching)
3. Skills which apply or receive elements from/to other skills (flame/ice charge, charge blow) are still affected by P resist and the skill being applied to (Rush, Blast, Power Strike) is affected as well. (See this post for more info)
4. Summons that are element-neutral (Summon Dragon, Gaviota, Octopus) "miss" on P1 and hit for half damage on P2
5. "P" immunity does not prevent status effects from neutral skills (Blind, Hamstring, Disorder)
6. Marksman's "Snipe" is not affected by "P" resist

 Methodology

1. Compiled and ran Vana on the newest rev (0.74)
2. Set Green Snail (100100) to "P1" elemAttr on the server and client (thus granting immunity)
3. Attempted to hit the snail with neutral element spells

 Proof is in the pudding

 Hitting other enemies with MClaw

 Hitting Green Snail with MClaw

 ...with Energy Bolt

 Power strike

 Level 1 Flame Claw

This is normal damage - no reduction

 Power Strike with Flame Charge

 Flame Charge with Charge Blow

 Star Throw from claw

Equips/stats:
Clean garnier
Subis
999 luk

 Lucky 7

Equips/stats:
Clean garnier
Subis
999 luk
Max L7

The following skills are unaffected by P resistance:

Hero: None
Paladin: None
Dark Knight: None

F/P AM: Fire Arrow, Poison Breath, Explosion, Poison Mist, Compo, Fire Demon, Elquines, Paralyze, Meteo
I/L AM: Cold Beam, Lightning, Ice Strike, Thunder Spear, Compo, Ice Demon, Ifrit, Chain Lightning, Blizzard
Bishop: Holy Arrow, Shining Ray, Bahamut, Angel Ray, Genesis

Bowmaster: Inferno, Phoenix
Marksman: Blizzard, Freezer, Snipe

Night Lord: None
Shadower: None

Buccaneer: None
Corsair: Flamethrower, Ice Splitter