here is a good stratagy for yunalesca, have everyone under zombie and dont use Aeons till her final form, then use reflect when everyone is on zombie, then she wont be able to cast cure or regen on your zombied characters also if you have Rikku's trio of 9999 it would be a cinch here sis a list of things that you can use to do Trio of 9999:

Bomb Core + Dark Matter
Bless Gem + Wings Disc
Lev 3 K S. + Wings Disc
Lit Mrbl + Dark Matter
Bless Gem + Gmbl Sprt
Lev 3 K S. + Gmbl Sprt
Drgn Scl + Dark Matter
Bless Gem + Undrdg Scrt
Lev 3 K S. + Undrdg Scrt
Arc Wind + Dark Matter
Bless Gem + Win Frml
Lev 3 K S. + Win Frml
Shn Gem + Master S.
Suprm Gem + Master S.
Lev 4 K S. + Pendulum
Shn Gem + Warp S.
Suprm Gem + Pendulum
Lev 4 K S. + Amulet
Shn Gem + Pendulum
Frpln Shdw + Dark Matter
Lev 4 K S. + Door Tmrw
Shn Gem + Amulet
Frpln Wind + Dark Matter
Lev 4 K S. + Wings Disc
Shn Gem + Door Tmrw
Dark Matter + Stmn Tblt
Lev 4 K S. + Gmbl Sprt
Shn Gem + Wings Disc
Dark Matter + Mana Tblt
Lev 4 K S. + Undrdg Scrt
Shn Gem + Gmbl Sprt
Dark Matter + Stmn Tonic
Lev 4 K S. + Win Frml
Shn Gem + Undrdg Scrt
Dark Matter + Mana Tonic
Door Tmrw + Door Tmrw
Shn Gem + Win Frml
Dark Matter + Twin Stars
Door Tmrw + Wings Disc
Bless Gem + Master S.
Dark Matter + Pendulum
Door Tmrw + Gmbl Sprt
Bless Gem + Warp S.
Lev 3 K S. + Pendulum
Door Tmrw + Undrdg Scrt
Bless Gem + Pendulum
Lev 3 K S. + Amulet
Door Tmrw + Win Frml
Bless Gem + Amulet
Lev 3 K S. + Door Tmrw
Wings Disc + Wings Disc
Bless Gem + Door Tmrw
Gmbl Sprt + Gmbl Sprt
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seriously if you dont want to use the level 4 key sphere you should its not then end of the world the stats are the main thing here and with trio of 9999 you can have Tidus do Blitz Ace and her 3rd form will be busted