First you have to get to Kyogre start to hit him with a good grass or electric attack
then weeken him with another atack such as thunder bolt or mega drain
now notice how his atacks are geeting weeker and weeker when that happens
use tackle or (the basic attack)all the way and he will ask to be on your team say yes and know you have one of the strongest pokemon in the game :note it works for groudon too