But going back on topic, I can't wait to catch all the available legendaries in Black 2. ^^
In Generation V, this was increased to x0.3 increments after each turn, so the catch rate increase is cut down by 2/3 of the time to the maximum.
@Kelleo, this is probably the reason you have trouble even with Timer Balls - they don't work as fast as they do in Generation V.
Also, there's one key factor which I think is what makes Cresselia harder than some Legendaries, and that's the surface you encounter it on - it's on a flat surface, so it doesn't get affected by any environment-based catch rate modifiers, or in the case of DPPt you encounter it in tall grass, which as far as I know does not have a ball that gets stronger if you use it in such an environment.
In this case, the best tips I can give you are this: Catch Legendaries at night so Dusk Balls work at their maximum, and use Timer Balls as best as possible. If you can, have status inflictions (sleep and paralysis are best, but if you can go for the Freeze when bringing health down. Until I used a Master Ball (as I was going for Hidden Power), that's how I caught my Latios) - this will give you more turns in general to catch the Pokémon and prevent it from Struggling itself for longer.
There's a lot of factors into catching a Legendary, and you have to pretty much be prepared for all of them.
@Wishmaker Jirachi;: Ah, I see now. Yeah, that makes a lot of sense. Thanks. :)