Pokémon Ruby, Sapphire, and Emerald – Why Are There Hearts Around My Pokémon?

In areas such as the Battle Frontier, the game puts restrictions on the Pokémon you can bring in. The Pokémon banned from the Battle Frontier in Emerald are:

• Mewtwo
• Mew
• Lugia
• Ho-Oh
• Celebi
• Kyogre
• Groudon
• Rayquaza
• Jirachi
• Deoxys
• Egg

In the competitive battling scene, Pokémon are categorized into different tiers. The most used tier is the Smogon Tiers. The tiers are Uber, OU (overused), BL (borderline), UU (underused) and NU (neverused). Only NU Pokémon can be used in NU battles. NU and UU Pokémon can be used in UU battles, all the way up to Ubers where all Pokémon can be used.

Note that tier lists change from generation to generation as Pokémon gain new movesets or counters.

So to answer your question, Latios can be used in the Battle Frontier.

I advise keeping the Sableye.

In 3rd Gen, it has no weaknesses. Not only that, but it can do more damage to psychic types.

Night Shade is not the best move in this situation, unless you grind up the Sableye, as it only deals damage equal to his level. I would definitely, however, advise grinding his level up, 41 would be a good level as he gets Shadow Ball there, which may lower SP. Def. It's also Super Effective against Psychic types and gets STAB. So a good move choice.