First of all, weaken it to around the YellowHP range. Then, use anything that makes it paralyzed or sleep. Then use a Dusk Ball (can be bought at Fuchsia City Pokemart) over and over until you catch it. Chances are, when its HP is yellow-ish or red, it will keep using Safeguard (which makes it immune to status problems), so parlyz it or sleep it first. Then you should be able to catch!
Okay I'm going to try to help you out as much as I can. First you want to have a ho-oh and lugia both about in the 70 level range, you want to weaken him with a couple hits from sacred fire, and then try to hit him with a pokemon that has strength to weaken him.this is my party
Ho-Oh 73
Lugia 57
Entei 70
Suicune 67
Rhyperior 80
and a Venasaur 72. I hope that this has given you a basic idea of what to use for some what
Ho-Oh 73
Lugia 57
Entei 70
Suicune 67
Rhyperior 80
and a Venasaur 72. I hope that this has given you a basic idea of what to use for some what