Penetration:
Penetration does exactly what you are probably thinking about, it bypasses a given amount of % resistances of the enemy. This even can go into the negative, so even on enemies with 0 resistance (which most enemies have) it will increase your damage.
Example:
- You have 9% Physical Penetration on your Amulet
- A enemy has 30% physical resistance
If you damage that enemy, his resistance will get calculated as if it has 21% physical resistance (you penetrate 9% of his physical resistance).
If the enemy has less than 9% physical resistance you will penetrate further than 0, so his resistance get’s calculated as negative, which increases your damge further.
Example with specific numbers
You use a skill that does 100 damage (every damage colaculation from player side already included) and you hit an enemy with and without penetration.
(Enemy has 30% resistance)
Without Penetration: 100 * (1,00 - 0,30) = 70 damage inflicted
With Penetration: 100 * (1,00 - 0,21) = 79 damage inflicted
In this example you deal ~12,85 more damage with the penetration.
To say if it’s better than %damage or crit multi is really hard to answer, generally and really depends on alot of things.
But usually on alot of well established endgame builds it’s probably one of the best stats, since it basically acts as some sort of “more multiplier”, since it elevates your whole damage output.
I would say in the majority of cases % increased X damage is worth less than crit multi and penetration, at least on builds that already have alot of %increased damage from other sources (such as other gear pieces, passives or idols).
Crit Multi can be really good to, but it also heavily depends on the build, how much crit multi is worth.
As a rule of thumb i would say both Crit Multi and Penetration are the best offensive stat. Crit Multi obviously only apply to crit builds, while penetration will work for everything, including ailments and DoT builds.
If you are doing a build that has several damage types penetration is definitely the weakest offensive stat, since more generic stats like “spell damage” or “damage over time” are better since they boost all damage types.
Shred:
Shred basically function the same way, but as a temporary debuff on the enemy, which also has a stack limit of 20 stacks.
Shred also reduces the enemies resistance on a given type.
Shred does shred 5% resistance shred per stack with a stack limit of 20 stacks.
On bosses and player shred does only shred 2% per stack with the same stack limit of 20.
Also Penetration and shred do work together.