reattempt in in a few more days
I managed to kill Dark Ixion on no sphere grid, had a super lucky attempt after around 15 attempts. The 2nd Dark ixion fight was extremely easy. All you have to do is watch out for his overdrives which KO's the party, which he doesn't use in the first fight. I would block the overdrive with Yuna summoning shiva, and if yuna couldn't get a turn when Dark Ixion had its overdrive full, then I would mix a Hyper Mighty Guard with rikku to put auto life on all 3 party members.
I also managed to beat Dark Ifrit, he has 1.4 million HP, he was pretty tricky to beat, his default attack is Meteor Strike and when dark ifrit uses it, it delays your turns if you survive it, he also counter attacks anything you do to him with a normal attack, the trick here was to summon an aeon, cast haste on it, cast shell, dismiss it so ifrit doesn't get a turn to attack aeon and delay its turn to death, then summon it again when ifrit nearly has his overdrive full, cast regen and use shield with aeon to minimise the damage of his meteor strikes and then survive his overdrive, and because he used the overdrives, my turn isn't delayed and i get around 3-4 turns in to attack with the aeon, 1 turn requiring you to heal back up, I would double cast ultima to deal the damage. and again, dismiss before ifrit gets a proper turn.
Quite a bit of luck is needed in Dark Ifrit fight just because you don't want him to constantly kill rikku or yuna when his nearly got his overdrive. For that reason you don't want to have both them in the party together when Dark Ifrit is around half way to his overdrive. You want someone like tidus or wakka to swap to the character when they get a turn if you can't summon with yuna, you mix a Hyper Mighty Guard with rikku to survive the Hellfire Overdrive. And while his overdrive is empty, mix a trio of 9999 with rikku on Wakka + Tidus any chance you get.
I also trained my anima to 255 strength with the aeons soul so I could deal around 380,000 damage with its overdrive to dark ifrit. Quite a bit of grinding, but it makes fighting some monster arena bosses alot quicker.
Defeating Dark Anima will be extremely hard as it has 8 million health, and its overdrive deals around 40k damage + take all your mana away, and his pain ability kills you even with deathproof, so it 1 shots any aeon. Which means you gotta rely on the Magus sisters to fight him, since Cindy can revive the others and use auto life, a very luck based battle, this will be the very last dark aeon i deal with