So, for all of you who want to fight the dark Aeons to fight Penance (I don't know why you would, but there are some complete freaks out there!) Here is a guide to help you on your way. You can't just charge at them unless you want to be royally kicked in the butt. If you follow the advice below you'll be one step closer to your goal.
Celestial Weapons
They are a must. I know they aren't the easiest of things to obtain but no CW, no victory I'm afraid. To even start the hunt you'll need the Cloudy Mirror which you may have obtained in the Calm Lands already. If not, head there now! Make your way to the exit on your chocobo, but don't actually exit the lands. Instead, turn until you see a path with a chocobo feather on it. Head up to it and press circle, across the bridge to Remiem Temple. Walking to the left and examine the sphere will let you head right and talk to the chocobo. A race will start and you have to get to the centre before the other chocobo does. Now, you have to race again to get the prize to customise your future celestial weapons so they can break the damage limit. Now you need to charge the mirror in Macalania Woods. Talk to the family there and head to their campsite. Now, head back and you will only see the parents and a sparkly path. Now you need to find their son! Head up the pathto the weird plant and you will find him. Charge the mirror at the plant and you're all done.
Now, to get the weapons you'll need to obtain sigils and crests for each member.
Tidus
Weapon: Caladbolg - race with the chocobo woman in the calm lands and beat her to obtain it. Head to the northwest section to examine a glymph. Use your mirror to obtain the weapon.
Sun Sigil: Racethe chocobo woman and obtaining a time of 0:0:0.
Sun Crest: Found where you fought Yunalesca. You'll find a chest there containing it.
Wakka
Weapon: World Champion - Go to the café in Luca and show the bartender your celestial mirror.
Jupiter Sigil: You need to have all of Wakka's overdrives and it is a prize in the Blitzball League.
Jupiter Crest: Found in the Auroch locker room. Examine all the lockers and you'll find it.
Yuna
Weapon: Nirvana - You need to capture every fiend in the Calm Lands and speak to the shop owner to obtain it.
Moon Sigil: Won after you beat Belgimine's aeons in Remiem Temple.
Moon Crest: Besaid Beach, swim out into the sea and it'll be in a little enclosed area.
Auron
Weapon: Masamune - You must have the Rusty Sword (it'll be in your key items, found where you beat defender X) and go to Mushroom Rock Road and find the statue of Lord Mi'hen. Insert the sword and examine the glymph.
Mars Sigil: Unlock the ten monsters in the creations (Monster Arena).
Mars Crest: Obtained on Mi'hen road.
Lulu
Weapon: Onion Knight - Go to Baaj temple and defeat the monster there, the weapon is in a chest underwater.
Venus Sigil: Dodge 200 lightning bolts in the Thunder Planes!
Venus Crest: Go to Guadosalam and go to the Farplane.
Kimhari
Weapon: Spirit Lance - In the Thunder Plains activate the three cactuar stones. Follow the ghost to the lightning tower.
Saturn Sigil: The Butterfly mini-game in Macalania Woods. You have to capture the blue butterflies WITHOUT touching any red ones.
Saturn Crest: Between the columns on Mt. Gagazet where you fought Seymore.
Rikku
Weapon: Godhand - On the airship enter the password GODHAND and you'll be taken to mushroom rock. At the end of the path there is the chest containing it.
Mercury Sigil: You'll need to complete the cactuar village quest in the desert.
Mercury Crest: Sanubia Desert west in a whirlpool.
Now that you have all the items, take them all to the weird plant in the Macalania Woods to complete your weapons.
You'll still need to max out your attributes for each character so they have these stats:
Strength: 255 Break HP Limit (30 Wings to Discovery), Auto-haste (80 Chocobo wings), Auto-Phoenix (20 Mega Pheonix) and Ribbon (99 Dark Matter). You can buy the slots from Wantz.With this prep you're pretty set for going kicking some butt!
Defense: 255
Magic Def.: 255
Magic: 20 (You don't need to max out your magic, plus it's a pain in the rear end)
Agility: 255
Evasion: 255
Accuracy: 255
Luck: 130 (No need to max out Luck)
HP: 86,000
MP: 999
Plus, your armour needs to have the abilities:
