The % of Zanmoto only rises to 50 if you Grand Summon him. In regards to the Dark Aeons, saying 'only Dark Jimbo should be the only one to cause problems' when you haven't stat maxed is lies. They will kill you faster than someone on a grassy knowl. Once you stat max, Valefor to Shiva can all be beaten pretty easily with Auto-Haste & Quick Hit. Once you get to Jimbo you need that Auto-Life. Dark Jimbo is pretty easy I found, the hardest would definately be the Dark Sisters. For an easier fight, try & play the mini game when you run away from them, it can get annoying but if you split them up they become a hell of a lot easier & you don't see Delta Attack which will slaughter you, no matter how much you have stat maxed. Penance is immpossible until you adopt the right strategic, then he is easy as hell. Once you have to fight Penance, if you want help, post here & I'll show you my strat for killing him.

With regards to Sin, I remember that battle well, I found it pretty easy. I beat him before maxing my stats.
Firstly equip all the Stoneproof & Confuseproof Armours you have, modify if you need to.
Easy Win would be to have Rikku, Lulu & Wakka, get Rikku to mix 2 Wings to Discovery & blast away, he's be gone in no time.
If you failed to do the Remien Temple Chocobo Race and got 3 chests then it's Plan B.
Start wit Wakka, Tidus & Lu. Tidus Casts Hastaga, Lu pounds away with her lvl3 spells & Wakka attacks with physicals. If anyone has learned Holy, replace Wakka with them. After 3 'Drawn to Sin' replace Lu with Auron to Armour Break it. After that pound away, he'll be gone soon enough.