For those who still have not catch 1 palkia yet. Can tried the 2 below methods as what my colleagues told me. Both of my colleague had 100% catch rate.
When not able to catch, can escape out and catch a few Pokémons outside before getting in to catch again. This method cannot be used if the time left on the raid is very low.
Try to sacrifice the first ball by just letting it row on the floor and then used the 2nd ball to catch it. My colleague told me he got 100% catch rate using this method........................
Good Luck To All............................