only Latios and latias can flee from battles plus the randomly appear making them almost immpossible to catch. they can be caught beforew the elite four
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The non cool thing about the battle frontier is you can't enter legendary Pokemon however you can go around that rule by having the following Pokemon: Regice, Registeel, Regirock, Articuno, Zapdos, Moltres, Raikou, Suicune, Entei, Latios, Latias. For some reason those are the only exceptions to the rule of legendary Pokemon maybe because the rest of the legendaries are overly powerful.
you dont
the first island in emerald is battle frontier
After you beat the champion in Pokemon Emerald, you can ride the ferry. Unlike in the Ruby and Sapphire versions, in Emerald you can travel to something called the "Battle Frontier." I don't want to give any spoilers for those who haven't finished the game, so all I will say is that it is worth the wait! Also, you can continue catching the legendary Pokemon and other Pokemon.
you can if you have a gameshark