No boss in FF8 can be considered hard when your characters have 1HP and Aura status. You can use Haste to make this strategy even more effective. I casted Aura and Haste on my party. As soon as Ultimecia used Hell's Judgement, I was able to pound her with constant Limit Breaks and finished her off before she was able to make her next attack. The same thing happened when Omega used Megido Flame. Ultima Weapon is extremely fast and cheap, but can be killed with one Lion Heart.

Hardest is Zeromus in FFIV (PS1) because he deals more damage with Big Bang than Rosa can heal with Cure4. Halfway through the battle, everybody except Cecil was KO'd and if I revived them, they'd just be easily KO'd again. So it was just Cecil, using Fight and an Elixer whenever his HP got below 2000. I ran out of Elixers before I finally beat him. It was almost a Solo Cecil challenge.

Next would be The Undying, I fought him with Lvl 45 characters on Active. (I think I was a little underleveled though) Almost all of his attacks could KO my characters in 2 hits, and he was really fast.

NeoXdeath was also quite hard, since he had multiple parts you had to defeat. In most boss fights, you had the advantage of many characters vs. 1 foe. Not here though.

Necron is hard, but not as hard as people think. Grand Cross doesn't actually do damage; it places statuses on your party (one of which is KO) and can reduce your HP down to 1-10. One time it even missed my party completely. After he uses Neutron Ring, he'll use Blue Shockwave 3 more times before the next Grand Cross. Blue Shockwave only reduces a characters HP to 1; it's not that bad.

All others weren't really that hard. Shuyin was the easiest.