The bold Pokemon must be in your party to activate the items or the NPC’s positive reactions. If you go without the legendary Pokemon, you’ll just get a generic conversation about the specific Pokemon and how you should speak to that NPC when you obtain that Pokemon.
Deoxys: Go to Ambrette Town and make your way to the Fossil Lab. The chunks of rock on the shelf are meteors. They will cause Deoxys to react and change form. It doesn’t matter which meteor you choose because Deoxys transforms in a cycle: starting from Normal -> Attack -> Defence -> Speed -> and back to Normal.
Giratina: Obtaining the Griseous Orb prior to obtaining Giratina is plausible, but in case you missed it, you’ll have to visit a hidden legendary in Terminus Cave. The sub-cave that houses Zygarde also houses the three timespace orbs. You are not required to battle Zygarde. If you feel unequipped to battle the behemoth, then you can grab the orbs and flee! Giratina needs only to hold the Griseous Orb to keep it in it’s Origin Forme.
Shaymin: Oddly enough, obtaining the Gracidea* flower is pretty silly given the restrictions on using Sky Forme Shaymin. Make your way back to Snowbelle City and enter the Pokemon Center. A female NPC near the Pokemart entrance will [!] when you start speaking to her. She’ll be happy to share a flower since she has a bunch. Keep in mind, if Shaymin is frozen, asleep, or if it’s night time, the Gracidea* will deactivate and result in Land Forme Shaymin.
Forces of Nature/Kami Trio (Tornadus/Thundurus/Landorus): Probably the easiest, but fly to Shalour City and then to the Reflection Cave entrance (it’ll be faster from this location). Enter the cave and head to the left. There will be a researcher waiting for you to bring her the trio. In my case, she specifically asked for Thundurus. She’ll give you the Reveal Glass* which will change the trio from their Incarnate Formes into their Therian Formes.
Kyurem: You’ll need Reshiram and/or Zekrom to obtain White and/or Black Kyurem. Head down to Kiloude City and take the stairs to the left of the Pokemon Center. You’ll come to two houses, enter the one on the right and speak to the punk-rock chick in the kitchen. She’ll give you the DNA Slicers* once you show her Kyurem.
Meloetta/Keldeo: These two are lumped together because the same NPC will deal with their form changing moves. Back to Snowbelle City, head left and up from the Pokemon center. Once you find the Abomasnow, head into the right house behind it. There will be a male NPC standing near one of the stairs and will teach both required moves for Meloetta and Keldeo. Keldeo will switch from Ordinary Forme to Resolute Forme after it is taught Secret Sword. Secret Sword is a Special Fighting-type move that is exclusive to Keldeo and will keep it in it’s Resolute Forme until the move is forgotten. Meloetta will need to use her move in battle to switch between Aria Forme and Pirouette Forme. The different forms also change her typing from Normal/Psyhic to Normal/Fighting (unique). Using Relic Song in battle will deal damage, change Meloetta’s forme between the two, and has a 10% chance of putting the opponent to sleep. Meloetta, however, does not keep her Pirouette Forme outside of battle and will revert back to her Aria Forme once the battle is finished.
*Items will be in the Key Item slot as they are not hold items.
Genesect: In Ambrette Town, you’re also able to get the Techno Drives to change the type of Genesect’s signature move, Techno Blast. Talk to the hiker in front of the meteors inside the Fossil Lab and he’ll give you all four of the Drives. Keep in mind, though, Techno Blast does not receive STAB for Genesect as it changes the type from Normal to Fire/Water/Ice/Electric, depending on the Drive held by Genesect (also means you lose out on a better hold-item).