I need a very powerful Pokemon to defeat Tate and Liza.
No, You can't catch rayquaza before the Seventh gym leader in emerald
'You have to beat the 8th gym leader.
Why do you choose an answer that is wrong? Dont you think it would have been to easy if you catch the lv 70 Rayquaza and use it against a gym with lv 40 Pokemon?
No, you can't. Sky Pillar only opens to you after you've defeated the E4.
No, you can only catch Rayquaza after you've beat the 7th gym and have gone to Sootopolis and completed the event with Kyogre/Groudon.