
I'll let others try and assess their potential for PvP, but generally, what I'm thinking is for the pierce rune, I would imagine the damage formula to be modified as such:
(Weapon Damage)*(Equipment Total)/(Monster Defense)/(1-Piercing Damage)*5*Random(0.7;1.3)
As such, a weapon that ignores 20% of an enemy's armor would work like this:
(Weapon Damage)*(Equipment Total)/(Monster Defense)/(0.8)*5*Random(0.7;1.3)
For the Piercing Rune, the rune would give weapons 12% piercing damage. At the wanted peak of Stage 5, the Piercing Rune would allow weapons to deal exactly 2.5x as much damage as they normally would.
For the Reflection Rune, the rune would provide the same reflection deal as the upcoming Reflection skill for shields. Reflection is gonna be a bit of a tricky thing to handle, so... Eh, how about a 1% chance of reflection per stage? That seems fair, right? I mean everyone's gettin' it.
I have made one monster specifically to allow players to grind Piercing Runes and Reflection Runes: the Crystal Crawler.

This monster would serve as a monster in the Red Leaf Hills caves, specifically serving as the strongest monster in said caves. It would appear in the 2nd cave area. As for its info, well...
Name: Crystal Crawler
HP: 320
Defense: 40
Power: 75
Piercing Damage: 15%
Reflection Chance: 10%
Weight: 2000 lbs
Eliapedia Entry: The Crystal Crawler is a formidable foe in the Red Leaf Hills' caverns. Its sharp, crystalline legs will pierce through skin, bone, and even armor when it attacks. Crystal Crawlers have fought against Xantis' forces when they tried to set camp in the caverns. The Crystal Crawlers successfully made Xantis' agents flee their attempted encampment, making the caves a relatively safer place for people to go through. Crystal Crawlers that attack humans fight against humans with the sole intent to test their strength against them. Perhaps something spiritual is behind their existence much like the Stalagmites.
Drop List
Raider Shoes: $92; 10%; 9.2
Watermelon: $180; 5%; 9
Raider Jacket: $459; 1.5%; 6.885
Raider Pants: $459; 1.5%; 6.885
Raider Hat: $617; 1%; 6.17
Pitchfork: $1920; 1%; 19.2
Raider Whip: $3439; 1%; 34.39
Compound Bow: $5600; 0.5%; 28
Back Tri Shield: $7413; 0.5%; 37.065
Goji Spine: $40022; 0.1%; 40.022
Tengu Bow: $158886; 0.01%; 15.8886
Striker Bow: $283656; 0.01%; 28.3656
Stage 1 Piercing Rune: $1; ???
Stage 2 Piercing Rune: $2; ???
Stage 1 Reflection Rune: $1; ???
Stage 2 Reflection Rune: $2; ???
Total Loot Value: 241.0712
As for what monsters drop what stage of Piercing and Reflection Rune, well...
Stage 1 Piercing Rune
Razor Ice
Rafflesia
Crystal Crawler
Stage 2 Piercing Rune
Crystal Crawler
Scorpee
Night Stalker
Auroch Beetle
Stage 3 Piercing Rune
Shroom Warrior
Scrap Bot
Moonchyllus
Stage 1 Reflection Rune
Razor Ice
Crystal Crawler
Policeman
Stage 2 Reflection Rune
Crystal Crawler
Tesla Crawler
Steel Block
Stage 3 Reflection Rune
Steel Block
Dome Shroom
Scrap Bot
So yeah, there ya have it.