Cobalion: Can be found in the Guidance Chamber of Mistralton Cave. Get it's health down to the red and then either paralyze it or put it to sleep because that will help out a lot. Next, have lots of Ultra Balls, Dusk Balls and Timer Balls. Timer Balls can be used after many, many turns have passed (the more turns, the better the ball gets, but it will never be a Master Ball), Dusk Balls are handy in dark places like a cave or at night. If it still seems to be taking forever, just have patience, this is a legendary after all, they usually don't come quietly unless you get lucky. :)
Terrakion: Only able to obtain after you have obtained Cobalion. Can be found in the Trial Chamber on Victory Road. Same thing applies here.
Virizion: Also only obtainable after you have Cobalion. Can be found in Rumination Field in Pinwheel Forest. Same technique applies here as well.
Tornadus: Trade with someone who has Pokemon Black or go out and buy yourself a copy and trade with yourself.
Thundurus: After you obtain the last badge from Opelucid City, head north out the gate towards Route 10. The person behind the desk there should automatically stop you telling you about a bad storm that's happening on Route 7. Fly to the nearest city and run to Route 7. Then after walking across those narrow bridges, as you pass in front of the house with the old woman and 3 other people, (one in which will heal your Pokemon) she tells you about Thundurus (white) or Tornadus (black). Head outside to automatically run into the Pokemon. (Or start heading east, then you will run into it automatically). Same technique applies when catching this Pokemon. The difference here is that Thundurus doesn't stay put. In fact, it will take off as soon as you encounter it. Never fear. Just head to the nearest electric bulletin and watch for a random strong storm to appear somewhere. Head to that place and use some move or ability to keep the Pokemon from fleeing. (For example, in past Pokemon games, you could use the move Mean Look or the ability Arena Trap or Destiny Bond)
Reshiram: Trade with someone that has Pokemon Black or get your own copy
Zekrom: I believe you get it automatically when you face N inside his castle. Not really sure since I haven't gotten that far yet, but that's my guess.
Landorus: Have both Tornadus and Thundurus in your party and head to the Abundant Shrine. Previously stated technique applies here as well.
Kyurem: The deepest part of the Giant Chasm. Can only access the Giant Chasm after the main story. Catching technique is the same here as well.
Victini: Obtain the Liberty Pass from the Mystery Gift and use it to board the ship in Castelia City. (the ship that's at Liberty Pier) Careful as this Pokemon is only level 15, but if you happen to knock it out, leave the room and enter again. It will be waiting. Again, same catching technique.
Zorua and Zoroark aren't legendaries, but they are hard to obtain because you can only get them from a special event. You can get Zorua by trading your special event Celebi to your White version and then visiting the girl who was reading a book to a boy in Castelia City. He will become the Zorua. (Also, make sure you have an empty slot in your party)
Zoroark is obtained by sending one of the shiny event legendary "dogs" (Entei, Raikou, Suicune) over to your white version. I don't remember exactly where you go to catch Zoroark, sorry. Or, you can level your Zorua up to level 30.
I'm pretty sure that's all the legendaries. Happy hunting!! :)
