You get Revival herbs in Eterna City or go to the Herb Shop.
Some good Pokemon are Torterra, Staraptor, Rapidash, Garchomp, Milotic or Gyarados, and Luxray.
First, defeat the Elite Four and get the National Dex by seeing all 210 Pokemon in the Pokedex. (You can see pictures of Dialga and Palkia in Celestic Town by talking to the elder after defeating the Elite Four). Then, go to Mt. Coronet. On 4F(2), which is up the waterfall in the cave when you pass the first snowy part, are the Adamant Orb and Lustrous Orb. You need both to make Palkia and Dialga appear. Both are level 70. Dialga attacks first and has Roar of Time, Heal Block, Slash, and Earth Power. Hit it with a strong neutral attack or double damage from Fighting or Ground moves, then gradually weaken it with Grass, Normal, Water, or Electric. False Swipe works, but does very little damage, so only use it once it is very low on health. Use a Quick Ball at the beginning of the counter (who knows, sometimes it works) then use Dusk Balls, Timer Balls, Ultra Balls, and if you have a very high level Pokemon, Nest Balls. DO NOT waste your Master Ball unless you have more than one (from lottery event or importing from Pal Park) or are lazy, they do not run when you see them and they keep respawning if you kill them, just defeat the Elite Four again.
After you catch or defeat Dialga, Palkia attacks. Weaken it with Dragon-type moves or a strong neutral attack, then reduce further with Fire, Water, Steel, or False Swipe. Give it Paralyze or Sleep. Again, try tossing a Quick Ball at the start of the encounter, then after weakening it, use Ultra Balls, Timer Balls, Dusk Balls, Net Balls (it IS part Water) and if you have a very high level Pokemon, Nest Balls.