Knock Yourself Out! The Goddess Beat the Final Boss in the Tutorial, So Now I'm Free to Do Whatever: Volume 1 by Supana Onikage

My name’s Karina—or at least, it is now! I used to be a nondescript, run-of-the-mill Japanese salaryman until I was whisked away to a divine realm where the Goddess of Time and Space told me I’d be living a new life in a new world—and as a girl, since apparently she prefers it that way. First she wants me to beat some final boss she has a grudge against, then she takes control of my body to show me how to use my new spatial magic powers in a tutorial fight... Wait, isn’t that the final boss I’m supposed to beat?! Less than in a minute into the tutorial and he’s dead?! And now she says the world’s my oyster. Seriously?!