Darkrai can be caught in any kind of Poke Ball, but Ultra Ball has 4x the catch rate by itself, which is pretty high.
Yes, you can catch Dialga without a Master Ball. Dialga can be caught with any kind of Poké Ball but Dusk Balls and Ultra Balls would be the better options.
If you're talking about what kind of balls are the best to use, I'd recommend Dusk Balls if you're fighting at night, since they have a 3.5 x catch rate. But if you know that it's going to take a long time to catch Ho-Oh, then I would use a Timer Ball, which is amazing if you're going to be battling it for at least 30 turns.
You don't need to do anything. Just enter the code. When you throw any kind of Poke Ball, it will be like a Master Ball, even if the Pokemon is at full health.
if you want my opinion, Gyardos is way better. It is a better battler than Milotic. However, Milotics usually have higher HP, but that kind of depends on the level you compare them at.
Any kind of pokeball can catch Mersprit, but it is best to use a Pokemon with mean look if you have troubles with it running away.
The best pokeball to catch registeel is a repeat ball if you have owned a registeel before trust me it will work wonders for you i caught registeel with a repeat ball without even weakning it.
You catch Cobalion with any Pokeball that is possible to catch it in.
master ball if you have action replay
a quick ball or a master ball
you catch him with a master
To catch Palkia, First Damage it, Use a Pokeball, Great Ball, Ultra Ball, Dusk Ball or any other kind of ball. Or just use a Master Ball.
Use some kind of Poké Ball! (i recomend Quick Ball)
depends on which game. IN Pokemon hg/ss the dusk ball is the best. In r/s/e timer balls and ultra balls. Watch out for fissure and double edge
A lure ball :)
masterball if you want to catch a really strong legendary pokemon. use an ultra ball if it is a strong regular pokemon.maybe legendaries
A tennis ball