Dark aeons will wreck your ***if you don't do extremely specific equipment and erase/perfect the sphere grid to max almost all stats, INCLUDING luck.
Also, whenever a boss is about to use their special move, you have to put out an aeon to eat the move because it will do 99,999 damage to everyone or just straight up kill them (Yojimbo).
It's the case for most of the dark aeons anyway, I know Dark Valefor is pretty weak, I think the next couple after it are as well. But overall, you grind for tons of hours to perfect the sphere grid, make certain equipment, and cheese the hell out of all the fights because everything one shots anyone/everyone.
Honestly, after doing it all I felt really let down. Beating Zodiark in 12 was much more satisfying than beating any of the aeons in 10.