It's sort of like Cressilia, except it doesn't runaway. Get heaps (and I mean HEAPSSSSSSSSSS) of Ultra balls and save before you go into the Harbour Inn because you can't get back to Canalave City without either killing or catching Darkrai. Then just cut it's HP down to the lowest possible number (you don't need to worry about killing it if you went by my last rule) and then just throw your darn Ultra Balls at it.*Hint:Paralyzing or making it sleep makes it a bit easier to catch.
the masterball is for any Pokemon you want, it can catch them without fail ( it will ALLWAYS work ) . you are supposed to use it for mewtwo though. :P
if you use an action replay and use the steal trainers Pokemon code and the 999 masterball code then you can catch it with a masterball and then its your's.
you use an action replay
Use you masterball on suicune because its impossible to catch it without a masterball it would just flee i cuaght mewtwo with a great ball but make sure you have lots of them.
Throw a masterball at it.
Event which is long gone, or trade from a friend.
Yeah it is possible to catch kyogre with out wasting your masterball you wont have to use it until you see latias and latias requires you to use your masterball.
weaken it and put it to sleep
you can, the catch is fairly easy even without a masterball although I woul use ultra balls if you arent going to use a masterball. I you knock out reshiram without sAVing before hand you can try again after the credits at dragonspiral tower
A fleeing legendary pokemon
try using dusk balls
=Its not possible but getting aceus is without action replay=
No.
Use something like mean look to keep it from running, then catch it like you normally catch a legendary.
at the end of the game
you need cheats or mystery gift
Use a quick ball and hope- but if it doesn't work it will flee