Here is a guide for legendary pokemon in emerald. and there level.

 

Name	Type 1	Type 2	Location		Level
Mew	Psychic		???????		????
Mewtwo	Psychic		Trade from LG FR		lv 70
Latias	Dragon	Psychic	choose Red		lv 40
Latios	Dragon	Psychic	choose blue		lv 40
Regirock	Rock		Desert Tomb		lv 40
Registeel	Steel		near lilcove		lv 40
Regice	Ice		North of dewford.		lv 40
Groudon	Ground		Near whismur cave		lv 70
Kyogre	Water		Route 129		lv 70
Rayqazza Dragon		Flying	Sky Pillar		lv 70
Deoxys	Psychic		Trade from LG FR		lv 70
Jirachi	Psychic	Steel	Trade from colloseum		lv 5
Artincuno	Ice	Flying	Trade from LG FR		lv 50
Moltres	Fire	Flying	Trade from LG FR		lv 50
Zapdos	Lightning	Flying	Trade from LG FR		lv 50
Raikou	Lightning		Trade from LG FR		lv 50
Entei	Fire		Trade from LG FR		lv 50
Suicune	Water		Trade from LG FR		lv 50
Celibi	Psychic	Grass	??????
Ho-Oh	Fire	Flying	??????
Lugia	Psychic	Flying	??????
Rurikio	Steel	Fighting	??????