You folks know about these, right?
For you AR/Pokesav/cheaters

, all you need are "Fateful encounter" versions of the three Pokemon. If handled correctly, their Trainer Memos should contain the characters "運命の出会い"

Kerudio event: Sekka Wetlands (from Route 8 )
Carry the three beasts in your party

Kerudio will learn a special sword move that's very powerful.

Meloetta event: Hiun City cafe (count third alley from Sky Arrow->Hiun entrance) Talk to the guitar player.
Meloetta will learn a move that allows it to transform into its Step Forme during battle.

Genosect event: Head to the P2 Laboratory off Route 17. You will be given the option to battle the scientist and receive some cassettes (Genosect's version of Arceus' plates) You can change the type of its signature "Technobuster" move this way.
These three, combined with the Victini, Zorua and Zoroark, make up all the known in-game events...I think.