Are the dark aeons vunerable to yojimbo's attack that cut's things in half?

You mean Zanmato? Nope, you can't do it to any of them.

With Dark Valefor, you probably should have max strength on Wakka, and then all you need is a prefect Attack Reel and you will kill him.

With Dark Ifrit, you can kill him with a perfect Attack Reel and Blitz Ace consecutively.

It's also easy with Dark Ixion, as long as you have Sleepproof, or better still Ribbon, and
Auto-Phoenix. This is because his normal attacks cause sleep in addition to
full break. You need to fight him twice. The first time you fight him, he will
only use Aerospark when his Overdrive is fully charged. Beware of his Thor's
Hammer overdrive during the second fight. Use Aeons to block if your HP is low,
and as his second incarnation is slower, quick hits should take him down before
he has a chance to use it.

For Dark Shiva, max agility is a must otherwise she will kill you before you have a
chance to attack. Use Aim several times to be able to hit her. Beware her
Heavenly Strike which is instant kill even with Ribbon armor, so rely on Auto
Phoenix. Use Aeons to block her overdrive as usual. Other than that, she is not
so hard.

Bahamut is a little tougher. Use all the tactics mentioned
above and use Aeons just before his Overdrive is full and shield because he
will do Mega Flare as soon as his bar is full circumventing your turn (i.e. bar
almost full, hits you, bar full, immediately Mega Flare before you have a
chance to retaliate). Auto-Life helps against his Impulse. His favorite tactic
is to do Impulse when his bar is full, then he immediately uses the full Overdrive
to do Mega Flare. My Auto-Life got all my characters back from Impulse just in
time to be killed :( Oh and Impulse has a 100% chance of turning you to stone, so Stoneproof, or
better still Ribbon is a must.

Yojimbo: First, you need to fight him 5 times by chasing the summoner back
to the entrance of the cave. You can save between fights but if you reload you
have to start all over again. I recommend going to save points to recover
between fights. His attacks all have a chance of instantly killing your
characters by doing 99,999 damage, and his evade is very high so do Aim MANY
times. Quick hit is the way to go with back up from Blitz Ace and Attack Reel.
His attacks can also do status effects but I had Ribbon so I am not sure what
they are. His Overdrive attack is an instant kill and even if you have
Auto-Life, you will not be revived so using Aeons to block is a must.

Anima is hard, and not just for the HP. Her Pain is an instant kill
attack which does not damage so Overdrive Comrade mode will not charge. Her
Mega Graviton attack will do incredible damage based on your max HP, so unlike
other gravity based attacks it can kill you outright if your HP is low. Her
Overdrive does incredible damage, not to mention draining MP. And for such a
huge lump she is very hard to hit, so once again do Aim like it is free, err
actually it is so just do it until you feel comfortable. As noted from her huge
HP, this battle will take a long time, but the standard tactic of lots of Aiming,
Quick Hits and using Aeons to block her Overdrive will win out eventually.

As for the Magus Sisters, I don't really know.

I did the zanmato to Dark Valefor......

Hey TeknoBlade is it possible to hit over 9999 with your characters ?
And how many overdrives has Tidus ?

I used Zanmato for most of the Dark Aeons. It should work... though I wouldn't recommend doing that as it will take all the fun away.

Robin: You need "Break Damage Limit" on your weapon and Tidus has 4.

Just cast holy on everything. Then when you fight that Yu Yeven floaty ball thing then cast refelct so he can't heal himself and do physical attacks and when he does his dispel thing then go back to holy but he heals 9999 everytime but if you average 22,000 like I did he'll be gone soon.
Only aeon that gave me problems was The Magus Sisters so they we're the longest to kill but everyone else was easy using Holy.
Hope I can help for once. :D

That Break Damage skill,wich item do you need to learn it ?

You need 60 Dark Matters, or you can just unlock the full potential of any ultimate weapon.

All Dark Aeons are easy with a Auron who has 255 in attack and a good amount of accuracy. Plus Break Damage limit.