He looks like a Behemoth. I fought him and died many times, but I finally succeeded XD!!!!!
He has 1 mil hp or so and yes...he does 99,999 A LOT. Haste, Auto-life, phoenix downs, mega-elixers, mega-ethers, x-potions, celestial weapons, mega-phoenixs, completed sphere grid, HP Break Armor, Damage Break Weapons...all that and it's still tough. This was my strategy.
[spoiler]
I had Wakka as my main attacker. When each character (Tidus and Rikku) would get a full overdrive gague they would use Entrust to send it over to Wakka. This way I could run his attack reels and do roughly 50K or so damage. If you get it perfect he will so 12 consecutive attacks. Now as far as defense...haste, and autp-life, also u can try casting protect and shell but it takes time (no protectra or shellra
)...Item wise i had a long of HP and MP boosting stuff like 3 Stars (1 mp per cast) and stamina tonics (double party HP) i got to use them a few times so I had 99,999 hp thru some of the fights. I got a lot of them from stealing and having the rare item abilites after a lof of fighting.[/spoiler]
I can't guarantee to win against Nemesis, its all about luck but you can try this and see what you think.