Turn on all multipilers and just keep collecting studs, if you have 100% just go to the stud fountain and leave your character to just collect the falling studs, helps if you have stud magnet turned on aswell, could even use a flying character to float in centre to get more studs with the use of the stud magnet.
Ok so i tried the above at the stud fountain and it didnt unlock, you need 2,000,000,000 i was already at 30,000,000,000 so i played the arrow dlc and unlocked it there as it seems to be collect the studs in a level and its not that hard to gain with all the red bricks turned on, you have a limitless number of baddies to kill just after you blow up the wooden tower, there are 2 tents that just keep spwaning baddies as long as you dont zip them up, baddies drop studs so there is an endless supply