Instructions
Decide which moves you will try to use against Latios. The moves Block and Spider Web will keep a Pokemon from fleeing throughout the battle; Mean Look will do the same but only as long as the Pokemon that used it is fighting; Sleep Powder will put the Pokemon to sleep for 1-5 turns; Wrap will do damage and keep the Pokemon immobile for five turns; and Stun Spore will reduce the Pokemon's ability to do anything to a 25% chance. Shadow Tag and Arena Trap are the two abilities which can be used.
Use a guidebook or an online database to discover which specific Pokemon have the desired move or ability or can induce the status ailment you wish to use. There are a great many such databases online, including "Bulbapedia," which is probably the most comprehensive and is searchable by move or ability name. Many Pokemon can use at least one of the moves from Step 1, but the only Pokemon that have the Shadow Tag ability are Wobuffet, Wynaut, Gothita, Gothorita, Gothitelle, Litwick, Lampent and Chandelure, and the only Pokemon with Arena Trap are Diglett, Dugtrio and Trapinch. Guidebooks and online databases will also tell you where to find each Pokemon.
Turn on your "Pokemon SoulSilver" game and, using the information you gathered in Step 2, find and capture the Pokemon with the move or ability you want to use. Most Pokemon encounters are randomized, but guidebooks and databases will tell you where they are most likely to appear. Some Pokemon with the moves or abilities listed in Step 1 do not appear randomly in "SoulSilver" and would have to be traded for using the Nintendo Wi-Fi Connection.
Level up your newly-acquired Pokemon by engaging them in battles. The Latios will appear at Level 35 and is very powerful, so you want your Pokemon with the move or ability you have selected to last long enough in the fight with Latios. Gain experience points by using your new Pokemon in battle.
Engage Latios in battle with your Pokemon with the selected move or ability. If you have a Pokemon with the Shadow Tag or Arena Trap abilities, you simply have to be in the battle with that Pokemon in your party. If using moves to prevent Latios from fleeing, you will need to make the Pokemon with the move in question your active Pokemon in the battle and use the move on Latios. The most effective strategy is to have at least one Pokemon with Shadow Tag or Arena Trap used in conjunction with at least one Pokemon with one of the moves listed in Step 1.