Dialgia

No you get Dialgia hp down and you use a pokeball but not the master ball

Absolutely not!

Dialga is really easy to catch with an ultra ball. Just save before the battle, so if you accidentally KO it, you can restart from just before the battle!
Take lots of ultra balls with you, and try to battle at night, so that dusk balls will work better, too... Take heaps to the battle... Like 30 of each or more if you can afford it... Try to get it's HP into the red without using poison/burn/confuse or any move that gradually does damage on dialga... It might take 30 turns of just throwing pokeballs at dialga to catch it.

There are Pokemon you will want later, such as mesprit and cresellia, that roam (like latias/latios in r/s/e etc...) they are random encounters, so it's more worthwhile to use your master ball on either of these two.

I used a master ball and it was easy I didnt even have to lower his hp and now I have him with full health!!

No your better off using it on mesprit which starts to roam about when you try to capture it.

No, save your master ball for a shiny Pokemon, easiest way to catch him is to play at night and use dusk balls.

It is better to use a bunch of Dusk and Ultra Balls after weakening it.

Definitely not! You can easily catch Diagla with Dusk Balls (if you battle him at night) or Ultra Balls. Just weaken him a bit, and possibly inflict a status effect. Don't waste your Master Ball on Dialga.

Instead, save your Master Ball for Cresselia. She's probably the hardest Pokemon in the game to catch, including the fact she'll run away whenever she gets the chance.

No, definitely not! Save your Master Ball for something better. Get Dialga's HP down and use a bunch of Ultra Balls!

By the time I'd beaten Cyrus, I was really low on health with only my Empoleon left. So... Really I had no choice I used it and actually I'm wishing I'd saved it for a lake Pokemon cos I accidentally beat up all 3

No, you should not. I used a ultra ball and caught it at full health. Dialga has a high catch rate of 30 for a legendary pokemon. Most legendaries have a catch rate of 3. If you fear that you will not catch it in single toss with an Ultra ball, weaken it first before tossing the ball. Also, there's formula that the game has for catching pokemon. Here it is:
((4x total hp of pokemon-2 x current hp of pokemon)x catch rate)/Total hp of pokemon+ status number
If status is asleep or freeze, the number is 11. If it is paralyze, poison, or burned, the number is 6. If there is no status problems, it is 1. This formula requires basic mathematical knowledge to calculate out the answer. Also, the catch rate depends on the ball you are using. For example, you are using a Ultra Ball, which has 2x the normal catch rate, to catch a Dialga, the catch rate will be 30x 2=60. Simple? Here's how the formula works: After you calculated out the formula, the games computes a random number from 0 to 255. If this random number is less than the number calculated using the formula, the Pokemon is caught. Here's an example:
Catching a Rayquaza at 1/2 it's original hp(let's say 200), no special status, with an ultra ball:

((4x 200-2 x 100)x Rayquaza has a catch rate of 3, but with Ultra ball, it becomes 6)/200+1(there's no status condition)

Work it out, you should get 10. So if the game produces a number from 0-9, Rayquaza is caught. Basically saying, the chances are low. (9/255 is around 3.5% chance of getting caught) Well, have fun!

You make me laugh. I'm new here-obviosly- but I pulled off a dare and caught it with a quick ball on the first turn. I thought it was hilarious, but it worked. I've always enjoyed using low powered pokeballs to catch anything. You know that mewtwo on blue? A pokeball caught mine. I would save before battling but I got mine on the 1st time trying.

Okay. Let me make this short qand easy. NO!!