the probability of yojimbo using zanmato is greatly increased if he has a full overdrive and you give hijm at least half of your money.

but personally i think yojimbo takes the fun and strategy out of the more difficult battles so i very rarely, if ever, use him. first you will need high stats.

lvl 2 and 3 key spheres can be gotten by beating the defender z's in the zanarkand ruins (lv3 is the rare drop) lv4 key spheres can be gotten by bribing the chimera brains in the calm lands (which are found behind the agency) 245,000 gil.

want a large amount of money? go to the omega ruins and defeat the mimic enemies with the gillionaire ability for 100,000 gil a pop.

now with that out of the way, you need to fight the creations in the monster arena under species conquest. these enemis will drop spheres which will put a +4 stat of the equivalent ability in an empty node on the sphere grid. do this over and over till ur stats are rather high (i recommend stats over 200 for the fight against dark valefor, with the exception of luck which shud be around 70-80-ish).

even with these stats, this won't be an easy battle. fill everyon'es overdrives before battle and have wakka use attack reels over and over by having other people entrust their overdrives to him. if ur attacks do 99,999 dmg:

12 X 99,999 = 1,199,988.

2 of these and dark valefor will be down. of course if ur not doing 99,999 a time it'll take a little more time but is still do-able.

i trust u have the celestial weapons? they are necessary for the dark aeons!?

dark aeons can drop armour with 4 slots, ribbon, and break hp limit. however this is rare. i recommend getting to a point where the battle is easy then simply beat him and if u don't get the armour you want do a soft reset and beat him again.

that shud do it. post agen if u want more info