This is impossible for me!

[Tavrobel]

Earth Eater has Celestial Armor, which reduces attacks by a random amount, usually a VERY HIGH amount. Not even attack reels will always do 99999, but it should do enough to knock it down. Normal attacks (Quick Hit, Mug) will do the most damage. When he falls back, he will be able to cast Flare, instead of Megaton Punch, which does 10k or less. You will quickly notice that he has reflect. So either Ultima and copycat, and hope it does 10k+ damage, or normally attack (normal attacks will begin doing normal damage). If he gets back up, entrust, and attack reels. It should kill Earth Eater. Don't use Hastega when he is still standing. You will lose too many turns on the user, and since he will do nothing but Megaton Punch, only Auto-Life and Pheonixes will help.

Greater Sphere is mostly annoying, since it will counter with Ultima, and when it attacks, it does Hydraulic Press, which does damage EXACTLY equal to your HP, unless you cast Protect and Shell. It will save you. Don't use magic, and all normal attacks will do normal damage. Use Potions as your main healing source, since you have Shell and it reduces healing effects as well.

Shinryu is underwater, so it will be Tidus, Wakka, and Rikku only, regardless of party set up. He counters all attacks with a normal attack, so cast protect. The only thing to worry about after Protect and Hastega is his attack causes Petrify. Otherwise, attack, and it should not be too hard. You will need to routinely defeat Shinryu if you wish to customize Break HP Limit on all your characters, but since all attacks do normal damage...

Nemesis.

Two words: (SPOILER) Use Zanmato.

But if you want a rematch, or beat him legitimately without losing ALL of your money and time on soft resets, having stats above 150 is a good place to start. His Celestial Armor will randomly reduce all damage, and Black Magic isn't worth using. If you missed Omega Weapon's Supernova for Kimahri, get it from Nemesis. The essentials; Auto-Life, Hastega. Customizing Auto-Remedy on armor will be useful beyond words. Ultra Spark causes around 7 Statuses, (including death by damage if your HP is below 20k) and some are painful to endure. Ethereal Cannon will simply cause heavy damage, but he uses it less often than Ultra Spark. Supernova = death, unless you can survive multiple Ultra Sparks. Auto-Life.

Have fun beating the enemy! MUAHAHAHHAHAH.
If you need more help, feel free to ask by PM or post.