FIGHT NIGHT CHAMPION

You can find the Online World Championship menu in the Live main menu. The bottom line is you must win. That should be obvious, but points are deducted when you lose. Not only do you have to win, but you have to perform well. Block, counter, stay on your feet all of these can count for or against you.

Your main goal is to obviously gain as much XP as possible.
There are numerous ways to earn XP in the game to level up your fighter
OWC- Winning or even losing OWC fight will net you anywhere from 300 to 400 points for winning (depending on quality of opponent) and maybe 100-200 for losing.
EA Store- This is the cheap (unfair) way of gaining xp, fastest and easiest. EA has xp up for sale in the ea store that you can buy to level up your fighter in OWC or legacy mode. If you do buy xp packs, I advice you to not use it on your legacy fighter because people have complained that they’ve lost their points in their fighter. Also, Legacy is so easy with adjusted sliders that you don’t even have to worry about it.
Online gym- You can gain xp points from fighting in an online gym with another person. This is a friendly match and doesn’t affect your OWC or your win-lose record in OWC. This is the safest way to get points, but you don’t earn as much as you would in OWC.

Not so much as a difficult achievement, but rather very time consuming.