First, the active player (or, if it's not an opponent's turn, the next opponent in turn order) chooses which creature they'll exile, then each other opponent in turn order does the same, knowing choices made before their choice. Then all those creatures are exiled simultaneously.

Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies. The spell itself won't count because it's still on the stack as you make this check.

If a player controls two or more creatures tied for the greatest power among creatures they control, that player chooses one of them to exile.

Use the power of each exiled creature as it last existed on the battlefield to determine how much damage is dealt to each opponent.