Trade from SoulSilver
The "SoulSilver" Latios is found roaming in Kanto randomly after obtaining the National Pokedex and leaving the Pokemon Fan Club or through the Poke Doll quest. You must obtain the Poke Doll quest from the lady in Saffron City who copies outfits. Travel to the house with the Bayleef in Vermilion City, and talk to the man next to the table. Go back to the outfit lady for another talk, then go to the man at the train station and ride to Goldenrod City, where Latios will be put on your map. Use the Wi-Fi trading system in the Pokemon Center to trade a captured Latios to a HeartGold game.
Action Replay
This will replace the first Pokemon in your group with a Latios. Put an unwanted Pokemon in that slot. Turn off the DS, and insert the Action Replay cartridge in the DS. When prompted, switch out the AR cartridge for "Pokemon HeartGold." If you have an AR Ultimate, insert the Pokemon HeartGold cartridge into its slot. Select HeartGold, select the "Add New Code" option, and the code 52247dac 582000c0, 12247daa 00004801, 02247dac e0021c39, 02247db0 0000017d and d2000000 00000000.
Special Event
Latios can be acquired in "HeartGold" with an Enigma Stone, which were given out at an official Pokemon Wi-Fi Event in 2010. The only way to get an Enigma Stone is to trade with a player who already has one. Once you have the Stone, go to the Pewter City Museum and talk to the guy who revives fossils. When you leave, you encounter a Latios.
Fighting Latios
Latios is at level 40, or level 35 if you meet him randomly in "SoulSilver." It is weak at a 2x factor to Bug, Ghost, Ice, Dragon and Dark Pokemon. He is resistant at a 1/2x factor to Fighting, Fire, Water, Grass, Electric and Psychic Pokemon. Latios is completely unaffected by Ground Pokemon. His statistics include hit points between 140 and 187, attack between 85 and 156, and defense between 76 and 145. The official Prima Guidebook suggests using a Pokemon with Block or another move which will not let Latios leave the battle.