In Emerald, once you finish the National Pokedex Professor Oak will call you and give you a ticket to go to masters island.There you can fight Ash and Gary

Well, they will have a double battle with you and the Pokemon they use are as follows:

1.Pikachu of Lv.85 and Nidoking of Lv.91
2.Charizard and Blastoise of Lv.95 both. and finally the main guns
3.Lugia and Ho-oh of Lv.100.

Well, to beat them, use a Groudon of Lv.100 which should know Earthquake, Solarbeam, Thunder+1 more attack.

A kyogre of Lv.100 which should know Surf,Thunder and maybe Calm mind and Sheer Cold.

A latios of Lv.100 who should know Psychic,Thunderbolt,Surf and Shadow ball.

This team is perfect against them.