Pokémon X and Y Walkthrough, Pokémon Move Sets: Tyranitar
(Please note that the recommendations below are largely made for in-game play. A Tyranitar in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents. Got a different moveset for a Tyranitar? It goes in the comments, and with all due haste.)
A standout pokémon from the second generation, Tyranitar may not seem a great idea at first glance. I mean, look at those weaknesses. That's a lot of angles at which to attack this thing. Fortunately, Tyranitar's strengths can often offet these weaknesses, and with a bit of EV trickery you'll find that this rocky dinosaur can take a heck of a lot of abuse.
Type: Rock / Dark
Tyranitar has inauspicious beginnings as a lowly Larvitair. It transforms into Pupitar when it reaches level 30, and will achieve full Tyranitarship at level 55. You can further evolve Tyranitar into Mega Tyranitar via Tyranitarite.
Dark, Fire, Flying, Ghost, Normal, Poison. Immune to Psychic.
Bug, Fairy, Fighting (4x), Grass, Ground, Steel, Water.
To make up for its raher extensive list of weaknesses, Tyranitar has some amazing stats. Attack comes first with a mammothine 134, allowing it to wipe out most opponents in one hit. Defense is second with 110, followed by HP and Special Defense at 100 apiece. Special Attack isn't bad at 95, aaaaand Speed... well, Speed's at 65. You can't win 'em all.
Mega Tyranitar doesn't change in unpredictable ways. Attack skyrockets, Defense gets even more of a boost, Special Defense jumps up a bit, and Speed gets a teensy bump. Overall, just an improved version of Tyranitar. Can't complain.
Sand Stream: Whenever the pokémon enters battle, it triggers a sandstorm that will remain in effect until the pokémon is defeated or otherwise switches out. Sand Stream is more or less Tyranitar's signature ability, and you'll come across these Tyranitars most of the time. Unlike most other pokémon with Mega Evolutions, Mega Tyranitar sticks with Sand Stream.
Unnerve: Opponents will not be able to use berries. This could have its uses, but... c'mon... Sand Stream. Only go with Unnerve if you're sick of seeing the Sand Stream animation over and over. This is a hidden ability.
Tyranitar is your standard wall, and it serves nicely in that capacity. Most of the time you'll want a moveset consisting entirely of combative moves. Stone Edge / Rock Slide cover rock-type STAB; Crunch is dark-type STAB; Earthquake, Thrash, Brick Break, and Aerial Ace cover the gamut of pokémon types. If you have a supply of Heart Scales you can also give Tyranitar Thunder / Ice / Fire Fang, all of which are good at targeting pokémon that would otherwise pose a bit of a threat to Tyranitar.
Though Tyranitar is suited to more supportive roles, you're largely better off keeping it as a wall. It's just too well suited to wiping out pokémon in one hit. That said, you can toss in a few moves to improve its destructive capabilities, namely Rock Polish and, through breeding, Dragon Dance. Equip it with one of the two and you should be able to outpace and outgun most pokémon with relative ease.
Desperate to try something people can't predict? Thunder Wave. Tyranitar is one of those lucky pokémon that can equip this move. Deploy it at the beginning of each battle and Tyranitar will usually be able to go first, despite a poorish Speed stat.
EV / Super Training
Most Tyranitars, particularly those that won't be using Tyranitarite, should focus on boosting Attack and HP. Tyranitar has solid defenses, but it just has too many weaknesses to stand for long. A hearty amount of HP can offset the damage, particularly from those deadly fighting-type attacks. If you plan on using Mega Tyranitar, you can either stick with basically the same build - never a bad idea - or, thanks to the extra defense, you can try to turn Tyranitar into a modest sweeper by boosting its Speed instead. I don't necessarily recommend this, but it's a possibility.
Catching a Tyranitar
You'll have to hunt down either a Larvitar or a Pupitar to get a Tyranitar, and you can find both of them in Terminus Cave in Pokémon Y. Larvitar appears in hordes; Pupitar will appear on its own. To further evolve Tyranitar into Mega Tyranitar, check the rock formation in Cyllage City's Pokémon Gym between 8 and 9 PM. It will only appear here in Pokémon X. (Tricky, Nintendo, very tricky.)