Tip: If you start training and I mean train seriously as in daily. Don't mouth off about it. This would do several things and none in your favor.
The other person would be aware and thus be more likely to put up his guard. The easiest opponent is the one who underestimates and doesn't take you seriously.
Also to brag about training, some people take it as an invitation to test you, you don't need tests.
This will take time, you won't get good over night, learn to understand not just physical mechanics but mental mindset, an unsure or unconfident fighter doesn't perform as well. Otherwise, go to someone of authority and report what's happening. Physical assault is a crime. By Law.