Final Fantasy X - Neslug at Low-ish level, No Mix or Aeons

Things are starting to get quite tough now. Th'uban was bad enough, requiring nearly an hour to beat, but I've reached the point where I'm so underleveled there is no dependable strategies anymore, I have to rely on the enemy AI to help me out.

I'm also somewhat at a loss for what an "appropriate" level should still be for most players at this point. As you can see, my Tidus/Yuna/Auron are sitting on nearly a full 99 sphere levels now. Since I haven't been grinding, I think it's safe to say most players should be higher level than me at this point. On the other hand it's easy to waste levels on the Sphere Grid, especially for first time players, so I'm not sure if there's any reasonable "average" I could predict anymore. Neslug requires one of every fiend in the game, which means you've been to the Omega Dungeon and Inside Sin, and probably fought a significant number of battles to find rare encounters. If you're significantly lower than my party here, or even roughly the same level, I advise capturing 10 of every enemy in those two areas to gain levels.

On a further note, if you're playing on the International version of the game don't use this strategy. When Neslug retreats into his Shell (or after 20 hits for 99,999 damage), Grand Summon Anima and use back to back Oblivion. If you're on the original release Wakka's your best bet, but you might want to throw some Strength Spheres his way.

Phase 1 is simple enough. Use a low-HP Auron to deal a consistent 99,999 damage. You want to hit Neslug for this value exactly 20 times to put him a single hit away from retreating into his shell. Any time he uses Slime it's a full party wipe, so you'll have to recover and rebuild with Auto-Life on your healer. Keeping Auto-Life on Auron will make it easier to get him to low enough HP to deal consistent max damage.

Once you've gotten 20 hits you need to kill everyone but Wakka, who should be in Loner Overdrive mode, preferably with Protect and 5x Cheer up (this MIGHT allow him to survive a physical attack when Neslug comes out of his shell at the end, which can prevent a Game Over). If your damage is around the same level as mine, it should take 6 castings of Attack Reels to break his shell. It's important to note that I'm doing a weapon switch between my turns. This is a Rank 1 action, which means fewer ticks pass on the clock so Neslug's Regen doesn't heal for as much. It's also crucial that Wakka have Haste.

I mess up on my 6th Reels and nearly blow the whole fight, but thanks to Yuna and Auron carrying their full Overdrive bar and Entrust I'm able to salvage (this was by design).

Once you've broken the shell Neslug gets a huge Agility increase, which makes Phase 3 way harder than Phase 1. This is where the luck factor comes into play. Unless your characters are substantially faster than mine, if Neslug uses Slime at any point during Phase 3 you're not going to be able to recover, as Yuna is just too slow to get another turn to renew Auto-Life. You need to hope for the best and burn him down as fast as possible.