The "Last Mission" portion of the game is not actually a direct part of FFX-2. If you didn’t play the International release, you will have never seen this part, since you couldn’t do the "last mission" outside of International versions, and now FFX-2 HD. Back then, the experience would differ depending upon when you entered the mission (since you could do so at any point of your X-2 gameplay from the title screen). In FFX-2 HD, it will seemingly assume you got 100% completion on your main-game playthrough.
So, what is Last Mission? The Last Mission is essentially an invitation given to Yuna, Rikku, and Paine three months after the plot of FFX-2, used moreso as a way of concluding the X-2 plot once and for all; throughout the tower, with every few floors, you’ll see cutscenes elaborating on this fact.
The mission itself takes place in Iutycyr Tower, and takes on a whole new concept versus the traditional FFX/FFX-2 gameplay systems; in fact, it’s relatively unused in the mainstream Final Fantasies. It is essentially a rogue-like game, now, in the style of games like the Fushigi no Dungeon SNES duo; the Pokémon Mystery Dungeon series; the Dark Cloud PS2 series if it were gridular; and the Chocobo Dungeon games.
That aside, it’s now time to describe the basics of FFX-2’s Last Mission. Like in most of the previously mentioned games, the movement system throughout the dungeon is entirely gridular and turn-based. You can walk one tile in any direction and it will take up a turn and, seemingly at the same time (though encountering multiple enemies can mess with that thought process), everything else will occur. The same is true about using an item (Triangle Button menu) or attacking (Circle) or changing Dresspheres or the like: anything you do, really, takes an action.
On each floor, your goal is to make it to the elevator to the next floor. (These were stairs in the previously-mentioned games: trust us, keeping the link between those and this can help.) All floors are randomly generated, except the boss battles every twenty floors. There are 80 floors in all, so that means four bosses: floors 20, 40, 60, and 80, with the ending following the latter.
You will use the X Button to attack. You’ll use the Left Analog Stick to move, and the Square Button in conjunction with it to rotate on the spot, or with L2/R2 to move diagonally more accurately. Press Circle and Select together to do nothing. (This can be good when you’re about a space apart and moving forward towards them would trigger you to get attacked: this way, you bait them.) Pressing Triangle brings up a menu, one aspect of which is items. There are four main item categories in Last Mission, and you can carry 20 total. You will see Potions (curative items or offensive ones you can throw), Books (super-special items with various effects), Dresspheres, and accessories. You can also change Dresspheres, use abilities, examine the ground, sort your items, and check your status.
Dresspheres in particular work slightly different in Last Mission, although their main purpose remains to give you abilities. They also give you new auto-abilities so long as you have those abilities and the Dresspheres are part of the set you chose (which maxes at five): one being the base and four being support. They augment your stats as well, like the main game, but Dresspheres themselves have a stat in their own HP. If all your Dresspheres hit zero HP, then your character is effectively naked and defenseless: from there, losing all of your HP means you lose. But, like most rogue-likes, there is no Game Over - you’ll just have to start over from the first floor. You will lose your items, though, unless you hit 70F.
The game interface is pretty simple. You move around and you’ll eventually encounter enemies; press Circle to bash them, heal mid-battle if needed, then move on. Nothing much will impede you in that. There are a few things that you won’t expect though, mostly the various traps. Most floors have at least one; they include warping traps, those that inflict damage, those that inflict ailments (we have only seen Poison, though), and those that can make your items be destroyed over time. You won’t see the traps coming, though, so tread carefully: they’re invisible until you step on them.
That all said, you won’t want to take too long on the same floor. Much like the previously mentioned games, staying around for too many turns (it’s either around 500 or 1,000 or something like that) on the same floor can make a certain enemy appear (the Founder). Like in those games, you’ll get three warnings before he does appear. The Founder will mostly serve to warp you from the dungeon, though he has his own function on Floor 77…
The bosses are just bulkier fiends (i.e. the battles are longer and you heal more often), and what’s there to walk you through is randomized except the Charmed Elevators.