Battle: Elven archers are attackers, neutral sorcerers are defenders. Archers use their special ability crack shot and "kill" sorcerers (not all of them), because they are resolved first. But sorcerers are using their special ability - undying. How many health will have the "killed" sorcerers? 0 or 1 (or 2)?

attacker resolves special results first, assuming the same initiative.

So sorcerers would die before special ability is triggered.

I'm with mateoo. As soon as the Sorcerer reaches it's maximum wounds, it is removed immediately. Thus, if the Elves were the attacker, their Orbs are resolved first, and if the orb is used to damage the Sorcerer, it's health is still only 1; it is thus killed and immediately removed. When it comes time to resolve the Sorcerer orbs, they are already dead, and the extra health does not help them.

+1 for the other responses. The sorcerers damaged by the archers will only have 1 HP each and die outright. Any surviving sorcerers will gain the orb advantage as applicable (there's no rule about fate cards being discarded when their units die, so it's not like the orb will disappear.)