Here's my strategy for Dark Yojimbo:

At the start of battle: Hastega, followed by Aim about three or four times, then Quick Hits. Each hit deals 99,999 damage, thanks to having characters with strength of 180 or so. Many attempts will end up missing, thanks to Dark Yojimbo's high Evasion, but hopefully enough will still get through. Quick Pockets is a helpful ability, since any hit from Dark Yojimbo may be instantly fatal, requiring immediate Phoenix Downs. Whenever his Overdrive gauge is nearly full, simply summon an Aeon. The Aeon will get to make one attack, then get killed by Dark Zanmato. Repeat until Dark Yojimbo dies...
The only risk is that your characters might miss too many times, meaning you run out of Aeons to sacrifice