This is, by far, the hardest not only in FFX, but in any game I've played! A lot of people agree, so here is how I end
up defeating her.
Ok, for the first round cast reflect on your attackers with Yuna (IE if you are using Tidus and Lulu, cast it on them).
Then just hack away! But get to full health before you finish her. (Waterga is Lulu's best Magic against Yunalesca)
The next round is more difficult. She will start by Zombifying all your characters. If you try to cure this status she
will just do it again. Don't bother. Only cure if you absolutly need to heal. When I founght her I used a Stamina Tablet on
one of my characters so their HP was higher. I didn't do it until the last round was half over, but doing it earlier might
be wiser. Anyway, for the second round, see if you can get Yuna into Overdrive and use Bahamut's Mega Flare. It should do
almost 15000 points of damage. If you can, upgrade Bahamut so he has a lot of HP. Then, after Yunalesca does her 3 attacks,
you should be able to do Mega Flare again. If you can't build his HP, let him die and continue to hack away at that ugly woman.
*NOTE* Don't bother using any other status than Reflect. Her regular hitting attack will Dispel any other ones.
The 3rd round is the HARDEST. You are going to want some sort of Armor that protects against Confuse, since her Mind
Blast will cause it. She keeps Zombifying you, but if you get out of Zombie and she uses Mega Death, you're thru! Try to keep
at least one character in Zombie (but I recommend leaving 2). She will use Curaga and Regen on your characters now, which
should be Reflected. Cast Shell on her so Curaga won't give her 9999 more HP, and Dispel when she gets Regen. Use Cheer to
raise your Str and Def, Focus to raise your Mag and Mag Def, and Reflex to raise your Eva (you will be able to avoid her attacks
easier). Just keep attacking and attacking. If you need to heal, do it one person at a time. She will eventually die and you
can be rid of her FOREVER! w00t!