Fight in October

Right. At 104 kgs (that is roughly 230lbs) and 5'11", unless you have freakishly high muscle mass, it should be relatively easy to lose. It might be worth looking at your BMI to see exactly where you are at.

I suspect you could do this with simply looking at your current diet, making adequate changes to it, and coupling that with a regiment of regular cardio, and possibly weight training unless you have an extemely high muscle mass content. In which case I would look into higher rep training vs. lower, and possibly bodywork exercises vs weight. Really depends on your where your body is.

You have roughly 3 months to lose 16 lbs, or 7kgs. For some it might be difficult, but again I suggest in your case it may not. Basically what you want is to be burning more calories daily then you are imputing, and given the amount of time you have, it should be fairly accessible and safe. As far as keeping it off, that really is subjective. If you can get into a routine that you are comfortable with and provides for daily caloric deficits, easy. If you do not, then it is self-defeating. So long as it is not simply water you are removing, then it is likely if you can follow good habits of eating and exercise, it will stay off. You should be able to eventually find equilibrium. It really is that simple.

This isn't very detailed, I know, but I will leave that to you or someone else who wants to fill in the blanks. If you are sincere about it, you'll do the research and find the way. But yes, very doable imo.