you buy guides and search online and hope for the best.
you can't catch it until you don't have a gameshark because its impossible to catch without gameshark
Nope not without a gameshark
You can't catch mew without a ticket. You can get it from gameshark or trade it from another game, but you can't catch it.
You can catch Deoxys at Birth Island. However, unlike other legendaries in the game, you can't catch it under normal circumstances. You have to use an Action Replay/Gameshark to be able to access Birth Island and catch Deoxys.
No not without gameshark, Action Replay or Trade
go to the next Nintendo event.
not without a gameshark
All if you have a gameshark
you don't need legendaries to catch other legendaries. just get a team of Pokemon around the level of the legendary you are trying to catch. weaken it until it is about to faint, and if you can put it to sleep or paralyze it, do that. then just keep throwing ultra balls until you catch it. just in case you don't catch, i recommend saving right before you start a battle with a legendary.
Mesprit Azelf Uxie Palkia
its impossible without gameshark or action replay.
The legendaries you can catch in Pokemon Ruby without trading or going to an event are Groundon, Rayquaza, Latios, Regice, Regirock, and Registeel.