I wanted to report that if you kill Guardian Vyn from the respawn platform instead of going down to fight him, then he will die and the “Defeat Guardian Vyn” checkpoint will never complete. This makes it impossible to complete the mission as you cannot progress beyond his arena area without the game recognizing that you’ve defeated him.
As Oscar Mike soloing the mission, I shot him through the drop hole of the respawn room, as well as fragcendiary nading him and calling airstrikes. He pushed me off the map earlier and with only 1 life left I decided to cheese him. Of course, I then got stuck as described above and had to quit the mission.