I got it!
I was lucky to join the first Mewtwo raid in my area. It was very exciting. About 30 trainers were present and most of us caught Mewtwo. We split up into two groups and we needed about 100 seconds to down her. Much easier than I thought. Mewtwo hits quite hard though (Focus Blast).
Edit (02/10): Changed attacks and powered up till breakpoint
Edit (10/10): Changed attacks and powered up till breakpoint