So, then, you're done with the Calm Lands? Are you done shopping, grinding, beating Belgemine, and riding cute li'l Chocobos? Then there is not much else to do. From the Calm Lands proper, go north along the far eastern wall and you'll end up on a bridge near Mt. Gagazet. Save as you prepare to cross it, for as you cross it...
BOSS - Defender X
BOSS ANALYSIS : Defender X is one of the most annoying bosses you'll probably ever encounter (outside of the Monster Arena): he hits hard and he takes a lot to bring down ably. Most of his attacks easily hit for 1,750 ~ 2,500 damage, typically enough to 1HKO/2HKO you with ease. It can use Blast Punch, which is just a counterattack that removes 50% of your HP. It can also use the very powerful Haymaker (expect 3,000 from it) and a basic physical (~1,500). It can also use Mighty Guard to nullify all elements against it once and to use Protect and Shell. It will also use Slowga with every five item or magical attacks that hit it.
MAIN STRATEGY : This battle will take a fair bit of buffing to finish properly; start with Tidus, Auron, and Yuna. Tidus should focus on Hastega , then lots and lots of Cheer for a while to buff up the party's Defense (since all of X's attacks are physical). Yuna needs to stick to Protect and simply Cura/Curaga thereafter, though she can be nice to Dispel Mighty Guard. And, finally, Auron is good for Armor Break . (Can't use Power Break due to an immunity. Crap.)
With all this set-up, there's not much left to say. Maintain what you can of it and be sure to revive and heal as soon as it's needed. With Armor Break in, Tidus will be good at throwing up an offense after those five Cheers, and so will Auron. (You'll want to stick to a physical offense as to not invoke Slowga. While Blast Punch is not appealing, Yuna should have ample time to cure it on her last turn before X gets his.) And Yuna's just there to heal and revive. Just batter him down, and use Aeons if you feel like it. (As a footnote, you can try to have Dark Buster take effect; the odds are only like 5%, though, so I don't recommend wasting time. However, the benefits are large.)
ALTERNATE STRATEGY - PROVOKE: As it so happens, the use of the Provoke ability creates a huge hole in the boss's offense. When you use Provoke, Defender X will only use - and ONLY use - Blast Punch on the Provoker. Blast Punch deals damage equal to 50% of your current HP, rounded up, meaning it can't kill you (1 HP * 0.5 = 0.5, round down = 0, 1-0=1) at all. So long as you don't remove the Provoker from the game somehow (like kill him yourself, use Summons, or the like), you'll be guaranteed to not lose!
ALTERNATE STRATEGY - TRIO OF 9999 : Yeah, expect me to note something like this in every boss for a while... Anyhow, if you got the 30 Wings to Discovery from Remiem Temple, you are in luck. Mixing two of them together yields Trio of 9999, turning all actions' effects to 9,999 damage or healing. That includes, for example, Yuna's Pray. This is a set 9,999 damage - nothing affects it, so... Anyhow, if you have Rikku Mix those two, Tidus can follow with Blitz Ace or Lulu with a nice Spell Fury of any kind (7+ casts) for a quick win.
After the battle, continue on to Mt. Gagazet. Don't forget the Cavern of the Stolen Fayth if you want a good sidequest!