@ObsessionUnbound
Defeat Volo and Giratina in the post-game
Return to Galaxy Hall in Jubilife Village
Talk to Cogita, who will give you information about their locations
Landorus: Ramanas Island, Obsidian Fieldlands. Weather must be sunny (unobscured sun on map).
Thundurus: Near the two large spires in the ocean, Cobalt Coastlands. Weather must be a thunderstorm (cloud with lightning bolt on map).
Tornadus: Bonechill Wastes, Alabaster Icelands. Weather must be a snowstorm (snowflake with wind lines next to it on map).
They're easy to find, but difficult to battle and catch. When you find one of the Forces, they’ll be surrounded by a wind barrier that repels Poke Balls. If they see you coming, they’ll quickly dart away, and if you try to chase them down, they’ll unleash wind attacks with tricky patterns that can knock you out in just a few hits. Not only that, but their preferred hangouts are teeming with aggressive Pokemon, as well as at least one Alpha, which means you’ll be drawing their attention as well as the Forces dart all over.
To break the shields, you need to successfully hit them with a throwable object (mud balls, snowballs, Poke Balls, etc) 3 times. This can be done whenever they’re standing still, either while they’re attacking you or when they haven’t noticed you. A great tactic for this is to pack a bunch of Smoke Bombs, then throw them down as you gradually approach to hide yourself. As long as they don’t see you coming, they won’t run away or attack. After you hit them with something, they’ll run a short distance, but as long as you keep the smoke coming, they won’t turn hostile.
Once you break their shields and they become dizzy, you can either throw a Poke Ball to attempt to catch them on the spot, or throw out a Pokemon and do it through battle. Incidentally, when the time comes for the catch, use a long-range Poke Ball like a Jet Ball; all of the Forces are at least partial Flying types, which means long-range Balls get catch bonuses against them.
After you catch the Forces of Nature, you’ll need to get their Pokedex entries to level 10, which you can do easily by getting into a few fights with random Pokemon. When the entries are maxed out, go see Cogita at the Ancient Retreat, and she’ll drop a reveal on you: there’s a fourth member of the Forces of Nature: Enamorus.
Enamorus can be found at the Scarlet Bog in the Crimson Mirelands, and unlike the other three Forces of Nature, it doesn't matter what weather it is. Use the same method to catch Enamorus, then max out its Pokedex entry.
Return to Cogita one more time, and she’ll reward you with the Reveal Glass, which you can use to freely swap any of the four Forces between their Incarnate and Therian formes.
