Base Resilience provides an innate reduction to damage taken from other players, and scales up based on your character’s level:
Base Resilience starts at 35%
Increases to 40% at level 40
Increases to 45% at level 60
Increases to 50% at level 70
Increases to 55% at level 80
Increases to 63% at level 90
Battle Fatigue reduces healing and shielding by 40% while engaged in PvP Combat.
Don’t know about unkillable. Most of the time, killing healers is how I win. Last time a healer was offering gold for duels, they went down after my second set of cooldowns.
I do wish the Classic folks would use the forum that’s explicitly for them, but even the devs post stuff about Classic here in General (or they did, maybe they’ve changed their ways). General Discussion should perhaps be renamed “Retail General Discussion” to clarify (assuming the devs want a clear distinction).