The Sun Crest can be found after defeating Yunalesca, revealed by descending 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 to obtain it is after defeating Dark Bahamut.
In the Final Fantasy X HD Remaster version, defeating Yunalesca earns the Overcoming the Past trophy.
Yunalesca has three forms, each having a separate HP pool. Whenever a form is defeated Yunalesca will transform into her next form.
Yunalesca's first form counters physical attacks with Blind, magic spells with Silence and Special commands with Sleep. On her turn she will alternate between a weak physical attack, which removes positive status effects—except Reflect—and Absorb, which inflicts damage equal half of the affected character's maximum HP and heals Yunalesca by the same amount.
Yunalesca will start the second round by using Hellbiter, which puts the party in Zombie status, and then will cast healing spells, such as Regen and Cura, on the party. She will often counter-attack with a weak physical attack with the same properties as in her first form.
Yunalesca will begin the third round with Mega Death, which kills all party members not afflicted with Zombie or protected by Deathproof. Yunalesca retains Hellbiter, Regen and her physical counter-attack from her previous form, but now has access to Curaga which she will use against a party member, and Mind Blast, which attempts to inflict Curse and Confuse on all party members.
Yunalesca's third form pattern of attacks is:
Hell Biter or Regen
Hell Biter Or a heal/Regen.
Hell Biter or Curaga
When an aeon is used against Yunalesca in any form, she will cast Absorb on them on every turn.
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 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. 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. However, the party must ensure at least one zombified player is on the field in case of Mega Death.
Having armor pieces with Death, Zombie and Confuse protection would be optimal, but the player will need to switch armor at the appropriate time by watching Yunalesca's attack pattern. To get Farplane Winds needed in customizing the ability Deathproof, the player can capture one of every type of fiend from the Calm Lands for the Monster Arena owner and he will reward the party with 60 Farplane Winds. This is especially useful for Yuna to immunize her to Mega Death even without the Zombie status so she can revive and heal the others. 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.
Another tactic for the second and third phases is to keep the Zombie status active for any participating party members and have Yuna bestow them with Reflect. This heals Yunalesca on her succeeding turns but it also helps her refrain from using Mega Death. The party must have good Defense and Magic Defense to minimize damage from Hellbiter. If Yunalesca attempts to put Regen on a zombified party member with a Reflect status active, and it bounces back to her, Yuna can Dispel it. This tactic can work faster if any of the party members have SOS Haste and SOS Protect.