Round 1: Black Widow decides to kill Thor no matter what during Age of Ultron final battle for no reason. Both have their AoU gear and Thor is in character, BW is bloodlusted.
Round 2: Black Widow has 1 day of prep, knowledge on Thor's location via a hidden tracker and access to all the gear she ever used during MCU. Thor is prime Thor and has both Stormbreaker and Mjolnir but is unaware Widow is after him. BW is bloodlusted, Thor is in character.