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gear to defend against or use the following skills? What affects its success rate when the skill is at 100%?
Kote/Kans
Blackjack
Thief poisons
dagger specs
for that matter, spear/mace/axe/etc specs
These are just guesses and observations mostly, I'll leave it to someone much more experienced to build on.
Kote/Kans
I believe the factors that effect the skill are your dex vs. enemy dex + your skill. There are probably other factors. When enemies were caltraps'd they seemed to fail saves more often. There are edges to help defend against these such as the 'Strong Wrists' edge.
Blackjack
Save vs. paralysis I believe, though I might be wrong. Note: enlarging yourself two sizes above the enemy means they cannot blackjack you. Unsure if enlarging one size would help, it may. I'm not an avid thief player. No doubt there are edges designed to help also.
Thief poisons
Some races; orc and I think dwarf and duergar are resistant to poisons. Hide of the Ancient green dragon also conveys resistance. Some Shifter forms were immune to poisons also. I'd imagine saves are made on Constitution.
dagger specs
for that matter, spear/mace/axe/etc specs
Depends on the skill.
In fact, against all warriors as a melee class you could pretty much follow the same general formula. Damroll/Hitroll to a decent level followed by Hp (varied depending on your ability to gather/base hp) followed by str/dex (varied depending on the spec in question.)
Cranial is affected by head gear. Drum I would guess by the material of the armor in question (similar to weaponbreak.) I'd gear for str/dex most against a dagger spec, spear, mace and then axe I think. Spear and Mace may be reversed however.