FIGHT NIGHT CHAMPION

This one is quite simple - keep using your XP on one skill unless it reaches the maximum level, and the achievement will pop.

I would personally recommend doing this in Legacy Mode, because the AI is not intelligent enough to look at your abilities, realise that you have one awesome punch and the rest fairly weak. This means you can get to the range your chosen punch is useful from, and pick your moment. When maxed out, you can stun or knockdown opponents with a single well-timed punch, so be patient and wait for your moment. Online, people tend to avoid your strength, and you'll want a more rounded development as a result.

I went for hooks to the head as my first maxed out skills, because it is a great counter punch when the opposition comes swinging. Just get a short distance from your opponent, put your guard up and wait for him to open up. When he does, swing that hook and take him down.

If you are doing this achievement as part of Legacy Mode, it is certainly worth putting points into the other hand of the same punch once you're done - i.e. if you've maxed out right hook to the head, put points into left hook. That way, you'll be in position to utilise your strength but if your opponent attacks from the other side, you've got an ideal alternative.

The XP curve for the abilities isn't as exponential as it is in some games, so you'll get this pretty quickly if you focus on one skill.