Interestingly, for me, the most difficult opponent was David Goon because he relies on dodging to defeat you and is not very responsive to react to it. I think Slippery Sally should be the first because she is more friendly with mistakes despite having more health and killing easier without the health upgrade than John McStroonie.
Overall, I had fun hehe! It reminded me a lot of the flash games that existed in abundance in the early years of the internet and about 20 years ago would have been very popular. I believe that by redesigning the dialog boxes for more intuitive navigation, balancing the enemies better, putting an angel as a secret enemy, separating the runs between rogue (the current way, where losing means losing your money and all upgrades) and story (so that you keep the total money earned in the fights) and polishing the response time to reduce input lag, you have a great time killer on your hands that could become something interesting in the long run. :) Thanks for sharing! I wish you a great day/afternoon/night!