Use a silver weapon or enchanted weapon
Cast Turn Undead or similar anti-undead spells
Use a weapon with a soul trap or damage magicka effect
Keep a magic weapon equipped, since normal weapons may not hurt ghosts
Use fire or shock spells if the ghost is vulnerable
Increase your weapon’s enchantment or use Fortify Attack effects
Summon allies to distract the ghost while you attack
Use potions or spells that boost your magic resistance and survivability
Avoid fighting unprepared in tight spaces where ghosts can overwhelm you