First you must have "Super Villain" status in all the villain levels.
Starting at the computer in Arkham Asylum Head out the door on the right.
Follow the hallway and around the corner you'll find two doors.
The one closest to the camera (your view) takes you to the 'Max Security Area'.
The other door is hard to see, but is in the corner next to the orange hot water heater. That door takes you to the 'Villain Trophy Room'.
In the Villain Trophy room, in the back corner will be a ladder that will take you up to the bonus level.
In the bonus level you don't have most of your extras available to you.
The goal is to collect a million studs, and try and beat your own times. (so ignore the clock for the achievement.)
In the first room is a little bit of a gym, smash everything, collect studs. You can play soccer and collect some studs slowly that way, put the ball through the goals by awkwardly pushing it, make sure it passes over the center button between goals. When the score hit 4-3 on the score board I got a trophy that could be destroyed for more studs.
But if you are in a hurry, once you have most studs in the first room, go through the door on the upper right side.
In this next maze like pacman level, smash things, get into vehicles, collect studs and eventually get to a million.
Getting into different vehicles spawns new paths of studs to collect.
The Poison IVY Ride on Plant, and ice cream truck can smash metal things that require explosives.
When you have collected a million studs the level ends. Most of the studs (85% or so) are found in the second room.