No. There may be more Pokemon that are better and/or more legendary later in the game. I suggest having a Pokemon about the same level as them and teaching it mean look and a sleeping and/or paralyzingly move. You will have to use mean look right away or else it will flee. Then lower it's hp and paralyze and/or put it to sleep then start throwing poke balls ( I suggest going at night and bringing 20 ultra balls and 20 dusk balls) until you finally catch it. I hope I helped :)
You should use a master ball.
i don't know. but i got riakou with mine.
Entei can be captured without the Master Ball however it's much more difficult. Once you find Entei attack it and it should run away. Now you must find it again when it appears its HP should still be lowered so now is your chance to capture it with any other ball you have.
by defeating Giovanni, the rocket leader, in the Silph Co. building. After ask the president and he will give you a master ball(you should save it for Rikou, Entei, Suicune, or Mewtwo)
One of the roaming Pokemon ex. Raikou, Entei, Latios, Latias
to catch suicune,mewtwo,raikou and entei
probably on mewtwo, suicune, raikou, or entei
You should use it on either entei, raikou or scuicune if you pick squirtle ull have raikou if you pick bulbasaur ull have entei and if you pick charmander youll have scuicune
Using a master ball
No. You should use your masterball on a wandering legend like Raikou, Entei, or Latias.
you ensure that you catch the entei, but it will be harder to catch the ho-oh using ultra or other balls
1 of the dogs because they flee and run all around the region