It is highly recommended that you get the Milky Way Map from Dairy Plant before trying to find this coffin as you actually need to take a hidden path to get to the correct room. Once you get close to the marker, it will point out of bounds, what you'll then need to do is look for a Silver Mirror located on the lefthand side of your view. This Mirror will then teleport you to where the coffin is. You must defeat ALL enemies guarding it as well as on the screen before you can pop the top off this coffin.
EDIT: If you wait on top of the coffin until the 30 minute marker hits, it will automatically open since all enemies are despawned for the reaper. Good for situations where the enemies glitch out of screen or you just cant get enough DPS to take them down. (Thanks to Naclimbe for the info)
A video on the entryway is shown below (but with some spoilers in relation to unlockable characters!!):