I cant remember how i beat her the first time round, but here's basically what is said in the Piggyback guide (which you really should get, its amazing):
'Yunalescas normal attack nulifies Haste protect and shell, so you cant rely on them for long. Portection against silence and darkness is vital (so equip some armour with it). Some attacks such as steal will be countered with sleep. However you should steal some Stamina Tablets from her (trust me these really do help, they double your characters max hp for that battle and will really help in your upcoming boss battles and especially against braskas final aeon). In her first form she shouldnt give ou too much trouble, but you should save your overdrives and aeons for the upcoming forms.
She will use Hellbiter which inflicts zombue on all your characters, and then use such things as regen to damage your undead characters. So have some holy water or remedies stocked up.
At thte start of the 3rd round shell use megadeath, killing all who arent undead. she might use this again later in the battle so be careful. (i would recomend keeping one guy permenantly zombified, and maybe even casting shell on him so yunalesca's healing spells dont harm him as much). She also uses mindblast, which causes confuse. Incidently, dispel will lift regen without altering zombie status. If you have holy: 1) after the battle train your asss off until you get it as many of the following battles are almost impossible without it 2) use it as much as you can.
and thats it. after the battle theres a sun crest behind where u fought her, so pick it up.
hope this helped
