Mistwalker Magina / FB 109 Bosses can be found in Lothranis/Heaven With Coordinate ( 352, 574 )
Demonic Dialectic / FB 109 Bosses can be found in Mommaganon/Hell with Coordinate ( 429, 594 )
but i think it more than 1 hour to be respawn cause its the dungeon boss so maybe the respawn time is same like waiting the dungeon to be reseted by its own when you out from it