I also was level 50 and took her down on my first try on normal without major problems. So either I was very lucky & you extremely unlucky or it's a matter of strategy. I used Seles and Kaie as magic damage dealers and Arden and Eileen for buffing, debuffing and healing. There is this water spell (I forgot the name) for constant healing (regen) you can get from a tarot card so always have it active and use large or medium potions if anyone gets seriously hurt. Don't underestimate buffing/debuffing as it's almost as effective in Didnapper 2 as it was in SMT Nocturne. Although Elize takes little damage from darkness still use Arden's Darkmatter Attack (dark or bone crystal) to keep her MDEF debuffed twice and use Eileen to also buff your MATK twice. Seles and Kaie will do a LOT more damage then.
Also make sure to go into the fight fully rested. After defeating Zhara you can go back and get captured for a last refreshing stay in the dungeons (yeah, I know there's a bed in the castle but beds are for losers). Just make sure you don't fight anyone while going back to not waste any CPs on the way (okay, you could also refresh CPs with items before going in but I never did that to save my chroma crystals for a fight I might need them in (but I never did).
Oh, I forgot: She can take down party members instantly if they get stunned. If that happens cure the stun ailemt ASAP and if it's too late stop attacking and focus on healing and freeing the person who got tied up as you need everyone. Eileen's spell to fill half of someone's ATB-meter can help a lot to make your party member with the highest rescue skill act quickly. (BTW you can also utilize stunning attacks to take down female enemies quickly although it doesn't seem to work on bosses (but sometimes on their support characters, I could take down both of King Byron's retainers that way without having to wear down their HPs)),
Hope that helps.
