You lack almost everything for even battle Monster Arena Monsters.
I think you find it hard with almosty all monster,right*(especially original monster)?
So the fastest way ,in my opinion, is using Magus sister to fight monster have power-up sphere(try to improve Yuna's power as Aeon power depend much on hers).
I suggest you get Strength sphere first!
And about armour,you don't need to restart if you don't get good one after a Dark aeon battle.Dark Jojimbo has everything you need.
Use Zanmato to beat him(do you know how to train Yoyimbo ? Give him 2000 gil each turn ,do this for 20-30 batle and he'll use Zanmato more regularly) for three or four times,then board airship then return to Fayth Cavern and fight him again(he'll dissapear only if you beat him near the exit),Break Dam weapon,Ribbon armour,ect await.
Good luck.