Answer:
there is a place called travler's tomb i believe its on the six island and its newly added to the fire red omega and you go through the pattern bush and then surf up to the cave
Things you need to bring
Strong Pokemon because there is a plethora number of strong trainers
plenty of ultraballs, like a lot and plenty of healing items like full heals and revives and some hyperpotions or moo moo milk is what i recommend which are cheap and nice you can find them at island 2
plenty of max repels if you don't want to run into wild Pokemon that are level 40 ish
From personal experience playing this game, i found out that the legendery birds are where they are like firered, and the three legendary dogs are in caves of the first three island. I don't think lugia or ho oh is in traveler's tome. If you care that is ...
hope this helps and good luck
do you know where in the travelers tomb ive been through the whole thing i found every legendary except latios and latias