This is a work environment apparently, not a school bullying situation. That's rare. Typically work bullying isn't physical in nature, but this one is. There's also a non-physical / verbal aspect to this, which is pretty typical in a work environment.
First, before anything I say next, I recommend just leaving the company. Get out of there. You don't have to make a fuss. Just find somewhere else to work where that guy is no longer an issue. You may want to tell HR about the guy after you resign. That's up to you, but anything you say which is negative will be recorded by them and can prevent you from being hired back to work there later.
Assuming you still want to work there, here's what I suggest:
If you have a problem with someone harassing, threatening, or assaulting you at work, first get evidence of it. Then go to HR with the evidence. And if HR isn't fixing it, then go to a lawyer.
Document, document, document. That means you record each time something occurred to you: dates, times, and what you observed. If there were witnesses, record who they were and ask them what they saw. Get a voice recorder and record everything they say. In most states in the U.S., you don't need to let them know you're recording their voice. So check your local laws to make sure you're okay to record without them knowing or approving it.
If you see this individual harassing others, document it and get those others to document it as well. Join with them. Usually bullies aren't just bullying one person.
Once you have enough evidence, go to HR with it and demand something be done about it. Document your meeting with HR. Give HR a copy of your evidence. Ask HR what specific actions they intend to take regarding this, and make sure to follow up with them once a week.
Continue to document.
If you're a member of an employee union, go to the union grievance committee and file a report with them first before going to HR. They can often get results much more quickly than going to HR.
If the harassment continues, get a lawyer and discuss it with him/her. There may be legal steps that can be taken to prevent it from happening. Sometimes all it takes is a letter from a lawyer demanding that the company take action to prevent their client from being harassed at work. Sometimes it takes the threat of a lawsuit to get them to comply.
Anytime you go to HR, know that HR is not on your side. They only care about minimizing risks to the company. So they're going to listen carefully to what you say and figure out what risk there is to the company in different scenarios.
Be warned. One of those scenarios HR considers will be your firing. They may decide you're the threat to the company and will then fire you. So, it is in your best interest to have documentation that will defend you from being fired unfairly. Make sure you have documents showing your raises over time, performance reviews, emails from management praising you, etc. This can help later on if they try to say you're being fired for not performing well. A lawyer will see that it's bogus and will have evidence to give to a jury showing that you were fired in retaliation for ratting on your coworker who was harassing you.
In most companies, HR is weak. They often don't want to or can't take meaningful action. So follow up with them. Ask them what actions they will take. Keep going to them and put pressure on them. If they are dragging their feet, present to them a list of all the dates and times you went to them, and tell them you've attempted to work with them but they appear to be unwilling or unable to help. Then say the next step is for you to go to your lawyer. That threat will put a fire under their feet. It escalates the risk level and allows them to take further action.
Make sure you get a voice recording of each meeting with HR, also. Again, make sure it's legal to do so where you're from.
Hiring a lawyer is not cheap. They'll probably want a retainer, which is a large fee up front to cover future work with them. It's typically $5,000. They can waive the retainer if you're in the low income category. Some may even work for free in some cases. But, know that if you do go all the way to filing a lawsuit with the company, then you can usually get them to settle out of court which will pay for your attorney fees and give you some cash in your pocket. So you may ultimately never pay a dime. HR knows this, which is why they will attempt to resolve it quickly before it even gets to a lawsuit.
As for fighting, come on, you're an adult, right? Do you really want to do that? Once you get in a fight on company property with a coworker, it will be easy to fire you. Your bully might not even be fired, but you certainly will.
And if you do get into a fight with that guy, guess who wins? He does. He's not as tall as you. But he's got a weight advantage on you. And unlike you, he studied something (boxing) which actually trains people how to fight. You studied something that was more theory oriented (karate and a bit of JKD). I don't think you're a fighter. And I bet you that guy is more of a fighter than you are. At the very least, he's much more aggressive than you are.
The old saying that if you stand up to a bully, he'll back down is rubbish. It's not usually the case. Bullies are bullies because they have unresolved aggression due to some childhood trauma. Not to mention their brains might be wired to be more aggressive to begin with. The fact that they pick on you just means you look weak to them. And they're usually right. Now, if you fight back and prove you're not weak, do you think they'll just back down? No. Not in my experience. Some will. Most will just go nuts on you.
You might be interested in learning about what makes someone a fighter, as opposed to just a martial artist, in my other answer here:
How does a person who just watches youtube videos to learn martial arts beat me in sparring?
So my advice is to avoid the guy from now on. Don't look for a fight. And if he's still bothering you, go through the process I described above. Or consider leaving to go work somewhere else if possible. Switching companies will also have the benefit of getting you promoted faster.
Hope that helps.