Fight Night Round 3

Go into career, then options. Set the difficulty to easy and turn OFF Illegal blows so that they can't be used against you. Then just use YOUR illegal blows continuously to win every fight. The announcer will say that the ref has warned you and you're hurting your career by being a dirty fighter, but it doesn't matter as far as achievements go.

It's pretty cheesy but you'll have 1000/1000 in around 3 to 4 hours with this method.

This will be the fifth achievement you will get in this game. 

Make sure autosave and illegal blows is set to OFF while 3 knockdown rule and HUD is set to ON and the difficulty is set to EASY.