Help! Sin and aftermath!

Potions, as much as I hate using them, it's your speck of hope. Keep your health up and watch your overdrive bar grow. Concentrate on Sin instead of sharing attacks (I think you can kill one or two sin spawn to lessen their numbers and prevent them from respawning. been a while since I've played). Stay alive and hit whenever you feel like it's safe (keep an eye on that atb thing to see who's turn it is next).
Also have you been using the sphere grid to boost stats?

 

Ok. I am a little bit confuse. Are you talking about the fight against Sin or are you talking about the fight against the Sinspawn Echuilles underwater or both?

 

OK! Let me see... (Thinks...)

1. Have you worked on Wakka? He can inflict the darkness status on the main boss. The four sinscales on both side will keep on coming so I suggest that you get rid of them, especially since you can get AP from them. If you overkill them, great.
2. For me, I think that you would have some hard time if Tidus and/or Wakka would be at a low health. Before you even think about ending the battle of Sin's tail, make sure that both of them are at full health.
3. Did you achieve Tidus' second overdrive? Slice & Dice will be very helpful for you. You can try to earn it on Besaid real early in the game after Yuna and the others joins you. Slice and Dice will be helpful for you to get rid of the sinscales and of the main boss is all alone, you may be able to overkill it.
4. If you have worked with Tidus, then you should be able to have the haste spell. This will allow you to react quicker. If not, make sure that you have the 20 potions that is obtainable on the S.S. Liki boat. Also, Tidus should be able to use Cheer by that time so have him use that ability as many times as you want.

Optional:
1. Did you get Valefor's second attack? You should. After you get done with the battle tutorials on Besaid, turn back and go straight to the village and find the small girl with her pet dog. She will be able to give you Valefor's second overdrive. A higher-level damage and it is easier for you to fight Sin's tail.
2. I recommend for you to work out with Rikku or Wakka under the water. You will be able to get some spheres faster and you can also level up Tidus and Wakka's defense. You can work out with Rikku, although you should know that Rikku will take a while and once she levels up, you can boost her hp.

Anything else?

 

I'll just type down what they have written down for the underwater fight in my Player's Guide.

As Wakka dives overboard to rescure Tidus, an underwater sinspawn emerges from the depths. A very difficult battle is about to begin!

Use Tidus's Cheer ability to improve both men's stength and defense, and use Wakka's Dark Attack to blind Echuilles. This eliminates its ability to drain HP from either character. You should ignore the Sinscales, although they continually inflict damage. Keep striking the sinspawn inan effort to end the battle as soon as possible. After using two Drain Touch attacks, Echuilles uses the Blender attack.

The Blender attack depelets a lot of your characters' HP. Continue to use Wakka's Dark Attack when needed. If Wakka runs out of MP, use an Ether to replenish it. Keep a close eye on the CTB Window on the right side of the screen. Use a Potion or Hi-Potion when a character has two turns in a row and is low on HP.