EDIT: This guide is only for those who have gotten "This...This is history" achievement as you will need to have gotten all of the artifacts to unlock this level.
By far the easiest level to do this on is "Warehouse" unless you kill yourself you cannot die, and all you have to do is shoot the large target in the back right corner of the room.
This level is accessed through the artifact room in the college.
To unlock the level you will need to get the key from the office, which you will need all of the different characters for (Explosives, Thugee, Scholar, and a small person). Then you'll need to put the crystal on the pedestal.
EDIT: To get all of the characters you need you will have to purchase them from the room on the other side of the hallway, once you have purchased them all of the characters you will need will be walking around in the room and you will be able to switch to whomever you need.
In order to unlock this level you will also need to find a number of the artifacts (I had all of them)
Go here for a great guide to get them all:http://www.xbox360achievements.org/forum/showthread.php?t=74...
Alright, once you get all that done for the "This...this is history" achievement you will be able to go into the Warehouse.
Pick a character with a shooting weapon, bazooka (which I used) pistol, machine gun, any will work.
Look for a very large target in the back right section of the room, once you find it continue to shoot it until you get to the 1 million stud goal. Don't worry, it will give you enough studs to reach 1 million it may take a few minutes though
As long as you don't kill yourself you will get the achievement, this is the way I got it so I have confirmed that it works.