I would strongly recommend to use healer with AOE heal. We had Sorcerer who specs in healing and Deluge and we were just fine, but Priest with instant AOE heal and Sanctuary will probably do better. The thing is, you will REALLY need AOE heal. Our party was Warrior, Sorcerer, Wizard, Rogue and Assasin (me).
Our equipment was weapons from Vador and blue/green armors from Wolves' Cave.

Now, about the battle itself.

• As soon as you will attack her, two tombstones will spawn. Break them and 2 NPCs will join you in battle. They don't really do much damage, but some people say that they have a buff that reduces incoming damage from her AOE skills, but we couldn't confirm this.
  
• She will also spawn Foxes that deal fair amount of damage and have about 2.5k hp, but they will not cause any trouble, if you bring them down quickly.
  
• When her HP reaches about 46k hp mark, she will spawn two copies of herself that have about 5k hp. It's better to get rid of them as soon as possible. They will hit you for about 50-100 damage, but at certain times (you will hear a loud bell ring or whistle) they will start hitting you for 500-800.
  
• From time to time she will use AOE stun attack, but since it has a short duration (about 1.5 second) and low damage, this shouldn't be a problem. Unless she stuns you the same moment she will cast her death spell, if this happened - you all will probably die.
  
• About the "Death" spell: when her HP reaches certain amount (61k, 41k, 15k, 9k, maybe i missed one) she will cast a DARK CIRCLE under her feet - you will have about 3 seconds to get out the circle and drag HER out the circle. If any of party members or the boss herself will stay in the circle after 3 seconds have passed - it will trigget AOE damage that will hit WHOLE PARTY for about 1500 damage and you will be wiped (althrouh Priest/Sorcerer can save you with Deluge/Sanctuary if you will have full HP when 1500 damage trigger happens. But in our case - Rouge, Assasin and Wizard were killed almost every time this happened).

NOTE: Sometimes, due a positioning lag, it appeared that she was outside the circle, but damage triggered anyway. So try to drag her away from black circle as far as possible. Don't forget to stay away from black circle yourself.


I hope this small guide will help you in battle against one of cutiest bosses of RO1/2 - Moonlight Flower!

P.S.
I guess devs were feeding her growth vitamins, because she was like 3 times shorter back in RO1.