In response to your question, SocietyzAntidote, you already have Shiva - she is the aeon obtained when Yuna goes to the Cloister of Trials with Seymour. In your aeon selection screen her character comes up as ???? - though you can still summon her.

Where vincent06's query is concerned, I'd just have to reiterate what everyone else said - use your aeons as shields and let off as many overdrives as you can. Try not to give her a chance to pull of oblivion.