I ran some Tomahawk tests to see if I could get a rough idea of what's going on~
I used a ton of STR + Meat, and used a 40 DMG Sword.
fSTR Cap = 12
DMG = 40
Base DMG = 52
First I went to get a control test using Sneak Attack on normal mobs:
Normal Crits:
155, 156, 150, 145, 159, 151, 161
Avg - 153.86
Avg pdif - 2.95
Then I went to find an Elemental~
35, 40, 38, 38, 40
Avg - 38.2
1 - (38.2/153.86) = ~75% melee damage taken
Then I used Tomahawk before each Sneak Attack~
69, 69, 66, 67, 63
Avg - 66.2
1 - (66.2/153.86) = ~ -57% melee damage taken
Then I went to find some Skeletons~
133, 132, 135, 130, 140, 136, 137, 142, 132, 141
Avg - 135.8
1 - (135.8/153.86) = ~ -12% melee damage taken
Then I used Tomahawk before each Sneak Attack on the Skeleton~
146, 141, 143, 132, 139
Avg - 140.2
1 - (140.2/153.68) = ~ 9% melee damage taken
From this data it follows that Tomahawk's Defense down partially removes a mob's resistance in proportion to it's base resistance. Mobs with higher base resistance will show a greater change /w Tomahawk as opposed to a Skeleton or Greater Mob which will show a much smaller change. The data size is small, so I think getting an exact % would be hard... but it appears to be around 1/3rd of base resistance is removed.
(Note: Before the 3.0 pdif hard cap was changed, I had data showing the base slashing resistance from skeletons was 12.5%. [896/1024] I'm fairly certian this was proven by other people.)
EDIT: Fixed a mistake in math
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