Go to the options menu and turn illegal blows off. Start career and throw illegal blows the whole time. You can buy illegal blows as well and recommended. Will take about 6 hours to complete if you go 37-0 with 37 knockouts.

Complete the following tasks to get GS points:
Burger King (100 pts.) - Win of The Burger King Invitational Fight
Dodge (100 pts.) - Win the Dodge sponsored fight
EA SPORTS (150 pts.) - Win of any EA SPORTS sponsored fight
ESPN FNF (150 pts.) - Win any ESPN Friday Night Fight event
ESPN PPV (150 pts.) - Win any ESPN Pay Per View fight event
ESPN WNF (150 pts.) - Win any ESPN Wednesday Night Fight event
Everlast (100 pts.) - Win the Everlast sponsored fight
Under Armour (100 pts.) - Win the Under Armour sponsored fight