Rusalka has an array of abilities, all of them dangerous. Her Bewitching Voice can charm an ally, requiring a party member to use a Remedy or Esuna. Aqua Regia deals a small amount of damage and reduces the target's P.Def. Her Seep ability causes her to dive into the ground and become untargetable, followed on the next turn by her most powerful attack, Dark Flow, which deals very heavy damage to the entire party and creates three copies of Rusalka. These copies have HP and no P.Def, so physical attacks will defeat them simply enough.
Give all party members Rebuff Lockets. A Freelancer is helpful, as it knows Examine, which can help determine the real Rusalka after she uses Dark Flow. However, a seasoned eye can determine the real Rusalka by simply looking at the enemy list. The one with 1 lower BP than the rest is the real one. Rusalka is weak to Thunder, so bring a Spell Fencer/Knight and a Black Mage to exploit this weakness. Default on the turn she uses Seep, as it deals a large amount of damage. She also knows Aqua Regia, an attack that lowers P.Def by 25% and deals a small amount of damage. It is recommended to take out the real Rusalka first, as the clones cannot use Seep, and therefore cannot generate new Rusalkas.