ze uber aeons

[Shin Gouken]

Are shiva and valefore easier than Ifrit?

Valefor yes but Shiva not. If you already defeated Dark Ifrit you should defeat Dark Valefor.

I don't plan on getting the magus sisters or anima... what i want to know is, is it possible to defeat dark anima and everything there after without them?

Yes. But you must remind that without them you will don´t have Nirvana´s Moon Sigil. Plus you will have 6 shields to deadly attacks instead of 8.

About Celestial Weapons, i recomend to acquire them for Tidus, Wakka and Rikku (and also level up this three characters).

I dont use my aeons as shields... thats what auto-life is for. Tidus and rikku are being levelled but wakka is almost untouched and i have no plans to level him up now. Ive given tidus break hp limit and break damage limit... what does his ultimate weapon have?