The battle takes place immediately after the battle with Sin, so the player should ensure that Tidus and Wakka are healed beforehand. While the Sinscales do most of the physical attacking, Echuilles will often use Drain Touch on one character, which can be negated by using Wakka's Dark Attack. Its other attack is Blender, which hits both party members for substantial damage. Making good use of Tidus's Cheer ability will raise his and Wakka's Strength and Defense enough to reduce the need to use lots of Potions.
After this fight, the party receives Sin (fin), Sinspawn Echuilles, and possible Sincales' AP, gil, items and equipment.
Did you know you can inflict Blind on Sinspawn Echuilles? It's true, kupo! Bring along abilities like Dark Attack or Dark Buster to turn out its lights!
—Dr. Mog's Advice
Sinspawn Echuilles appears as the boss of S.S. Liki in Final Fantasy Record Keeper. It is accompanied by three Blue Sinscales.
Attack: Deal physical damage to one target.
Blender: Deal moderate water damage to all targets.
Drain Touch: Deal physical damage to one target, gaining a portion as HP.
As per Dr. Mog's intel, it would be wise to bring along someone with a good affinity toward Celerity skills and try to hit this squidball with Dark Attack or Dark Buster. The latter has a much better chance of succeeding, but requires Lesser Orbs to synth, which are rare at these early stages of the game. More importantly, Echuilles' Blender could send the party to an early watery grave quickly, especially when followed up by its three Sinscale minions sending out Spines. Take two of them out to level the odds, then pound the boss and finish the third for the win