I think Ultimecia is teh 2nd easiest boss in FFs...(1st is Yu Yevon---he like an itsy-bitsy spider thing and very silly). If you have Lionheart and max. HPs for all characters, no proble to defeat Ultimecia...
I think the toughest boss in FFVIII is Omega Weapon. Still, if you leveled up very well it is hard anyway!! I was Lv. 100 and HP 9999 of all characters, and still I needed to beat that Omega for 30 minutes...Even I had Hero and Holy War too...But when you beat it, you take Omega Proof, and this was an honor for me to read it (trust me, it makes you happy)...
Anyway, I thought in my very early play times of FFVIII, I can beat Omega when ı was low-level..But its just a dream. Don't think about it.
Still I can't beat Necron well in FFIX...