Well, are you right up to Palkia, as in have you beat Cyrus and you're about to fight it? If so, Palkia isn't actually that hard to catch, its catch rate is 45, 15x more than other legendaries. Get it to as few HP as possible using moves such as False Swipe, Night Shade or Seismic Toss, just like any other pokemon. Try to throw a status condition on it if you can, like sleep or paralysis. Don't use poison, since that will knock it out. Once it's weakened and has a bad status, just start lobbing balls until you catch it. To make it a bit easier, use Dusk Balls, since they're 2x as good as an Ultra Ball. One thing to be sure of, DO NOT use your Master Ball. The Master Ball is better used on a runner like Cresselia or Mespirit, as they're much tougher to catch than this guy. Even if you don't have many balls, keep resetting and throwing balls until you catch him...it's possible to catch any pokemon in any ball if you're lucky enough (I got my Registeel in a Poke Ball).