Zanmato works on all monsters. On bosses, the money requirement is much higher, but since you have the Int. version (the wussy version), you can pay Yojimbo a much lower amount that in the NA version. The lowest reliable amount I've heard thus far is 100k gil.

There are some instances of Dark Aeons, where you can walk very carefully, so that they don't see you, but I doubt this is a viable option.

Fighting Arena monsters is a good way to get the items that you need to customize your equipment with the proper abilities, and you get loads of AP per defeat, and equipment that will make the rest of your life easier, or can sell for very hefty amounts. (8k, 10k, 50k AP, respectively)

To defeat the Dark Aeons, you can either Zanmato over and over, or have stats above 150 in all areas, except Luck, and face them head on.

Fortunately for you, you can still get Wakka's, Rikku's, Lulu's, and Kimahri's Celestial Weapons and Sigils. If you need more help, PM or contact me on AIM.