I'm fond of the monster arena bosses.

Kottos is really fun to fight especially when you're at low levelsxD

It reminds me of the ff2j way of levelling.

'Auron! You're sentineled, protected, and hopefully he'll look at you!' 'Yes ma'am Yuna, We'll survive this!'
*kottos hits rikku instead*
'Oh noes!!!' 'Hurry m'lady, phoenix down!'
*raises rikku*
'Oh man, what happened to my head?' 'He got you. Here, I'll distract him so he'll attack me next!' 'AURON NO!'
*auron attacks kottos*
*kottos counterattacks auron, taking his HP to about 25hp*
'Oh noes!'
*kottos' turn comes again. Throttles auron soundly, auron ko's*
'OH NOES!'

xD And that's the way it happens so many times.

Of course later you have sufficient defence that you can just smack him, but still!