UFC Undisputed 2010

This opportunity will come during the Career Mode. For me, the easiest way was to do the following:

1) Win the Championship in your weight class.
2) Decline the chance to move weight classes.
3) Defend your title 5 times.

After your fifth defense you will be offered the Champion vs Champion fight. Save, just in case you lose and once you've won, the achievement will unlock.

*I declined the weight class move to save the time of having to win that title and then defend it 5 times, good luck.*

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In career mode, you earn credibility ("Cred") for completing matches, sponsorships, etc. Once you are a champion and earn 50,000 Cred, the opportunity for this match will occur.

For me, my 19-0 Middleweight Champion received an offer to fight Light Heavyweight champ Forest Griffin when I had 50,019 credibility. The achievement popped after I beat him and finished the post match interview.