The pokemon you defiantly need is a Kyogre/Groudon over level 55 and five other pokemon at least level 42. Go inside the Sky Pillar and use dig. You will be at Rayquaza in no time. Make sure that you have 10 timer balls and 8 Ultra balls. Save before you get to Rayquaza and get him down to low health when in battle with Kyogre by using ice beam only once. Use up all of your Ultra balls and then use a timer ball. This should work because it worked for me.

PS: you must have bee ten the elite four before getting to the Sky Pillar and if you don’t know where it is, it east of Pacifilog town(route131) and when in the middle of route 131 go north and find a passage and then you will see a cave and that is the Sky Pillar.