I take your description to be a summary of the defense's argument, made in Washington courts. Betty is guilty of second degree murder, and since it's not reasonable to believe that a ghost, which does not exist, was trying to seriously injure or kill her, she will not succeed in her attempt at a self-defense defense. The jury concludes that at the relevant instant, she intended to kill the ghost.
She has committed a Class A felony, which carries a maximum penalty of life in prison and $50,000 fine.
The judge works through the guidelines, also there is a hearing to gather pertinent facts. We then need to know if she is a repeat offender: for simplicity's sake we assume she has no bad marks on her permanent record. She actually did it, this is not an attempt, conspiracy or solicitation. Betty's best hope is an "exceptional sentence", one outside the prescribed range (lower than normal, due to mitigating circumstances), allowed under RCW 9.94A.535 if the court "finds, considering the purpose of this chapter, that there are substantial and compelling reasons justifying an exceptional sentence". There is no mandatory minimum sentence for second degree murder, however since she committed a felony with a firearm, per RCW 9.94A.533 she earns a 5 year sentence enhancement. The offense level is XIV, we assume an offender level 0 (otherwise squeaky clean), the standard sentence is 123-220 months, but add 60 for the gun.
The judge then considers possible reduction of sentence under RCW 9.94A.535, being influenced by the claim that the victim was an initiator, the defendant made a good faith effort to "compensate" the victim, perhaps committed the crime under duress, coercion, threat, or compulsion insufficient to constitute a complete defense but which significantly affected his or her conduct, demonstrated a capacity to appreciate the wrongfulness of the act (and was not drunk or on drugs). The judge is aware than the prosecution (or defense) can appeal any reduction in the sentence below the minimum. Some reduction is thus possible, but reduction to a token sentence would not survive appeal by the prosecution. Suspended sentences are not possible and the first-time offender waiver is not available for murder.