The Sun Crest can be found after defeating Yunalesca, revealed by going down the end set of stairs. In the PAL, International and HD Remaster versions, if the player does not obtain it upon her defeat, the next opportunity they have to obtain it is after defeating Dark Bahamut.
Yunalesca will start the second round by using Hellbiter, which puts all characters in Zombie status, and then will cast healing spells such as Regen and Cura on the party. Zombie can be lifted with a Remedy or Holy Water, and Regen can be removed with Dispel.
The aeons are not terribly effective as Yunalesca will use Absorb and take all their health; however, a powerful Overdrive is always useful. Instead of killing her outright, Yojimbo's Zanmato will only cause Yunalesca to enter her next form (unless she is in her third form already).
Yunalesca will begin the third round with Mega Death, which kills all party members who are not afflicted with Zombie. Yunalesca will later use Mind Blast to inflict Curse and Confuse.
The fastest option assuming one doesn't care about everyone getting AP is to steal four Fire Gems from Flame Flans in the Calm Lands and ensure Rikku can mix Trio of 9999 (Quartet could work, but it would need to land on someone who has the ability Use). When facing Yunalesca, Rikku must mix Trio of 9999 and throw four Fire Gems for a quick win.
Before the battle, it is useful to have Yuna and Rikku reach Overdrive, along with any aeons. It is beneficial to customize the ability Auto-Med (or find an item) on either Rikku, Wakka, or Auron. When the battle begins, the trio should be in the starting line-up and steal a Stamina Tablet from Yunalesca. It can be used in mix with a Potion and to get Mega Vitality, which will double everyone's hit points. Yunalesca's greatest weakness is her lack of strong physical attacks, and Wakka and Auron's 8,000-10,000 hit points functions nicely for offensive purposes.
Yuna is handy for Reflect and Esuna. When the second phase starts, the party should continue the same strategy, mixing in Overdrives, although Yuna is needed to Dispel any Regen that might land. Whoever is the Auto-Med can function as a curer and use Holy Waters to cure the Zombie. A good follow up is to have Yuna use Curaga afterwards. However, the party must be sure to always keep at least one Zombied player on the field in case of Mega Death and the rest of the battle should pass easily.
An alternate, and perhaps simpler, strategy is to capture one of every type of fiend that exists in the Calm Lands for the Monster Arena owner and he will reward the party with 60 Farplane Winds, which can be customized into the Deathproof ability. This is especially useful for Yuna to immunize her to Mega Death even without the Zombie status so she can revive and heal the other characters. It is also possible to use Auto-Life to survive Mega Death by either casting the spell or by using Rikku's Hyper Mighty G.
If Yuna has learned Holy, it is a good time to use it.