After completing Mission 4: Investigate the Old Mansion!, the player will have access to the past, from which they can take 23 Missions to capture boss Pokémon within a time limit. A 24th Mission could be obtained with access to a Wi-Fi distribution that ran between 2010 and 2011; the mission is still playable on any copy of Guardian Signs that obtained it during the distribution period.
These Missions were intended to use local wireless multiplayer to support up to four players completing the Mission simultaneously. The Missions' maps often have paths (some of which lead to unique Pokémon or helpful power-ups) that can only be unlocked with two or four players, some Styler mechanics are introduced that allow the Styler to be powered up if multiple people are trying to catch the same Pokémon at the same time, and the fact that the Missions' goal is largely to catch Pokémon means that time is obviously saved if multiple players can capture different Pokémon at the same time.
That having been said, with the exception of the Wi-Fi exclusive mission, all Missions can be completed by a single player. The single player will have to do lots of grinding to get to a position where they are able to the expected work of two or more players within the given time limit, and some Pokémon are completely unavailable due to their requiring multiple players, but it is possible for the solo player to at least defeat all 23 regular Missions.
The pages here attempt to consolidate data on as many parts of the multiplayer mission experience as possible. The focus is generally on completing the missions as a single player, and to that end, I will be including guides and videos with the intent of showcasing strategies, maps, tips, and anything else that I think the single player could be armed with going into this experience. However, data for everything will be provided, even that which is multiplayer-exclusive.
My Missions pages will show descriptions and videos provided by other players that have the ability to record DS footage and gameplay, and have more experience with these missions. Currently, coverage comes from this Smogon thread by WarriorGallade, who provided maps and data sources for all the missions and suggested strategies for clearing all of them. His work is used with permission and I link his original posts where possible.
Unless otherwise stated, everything in this section is a work in progress.