Dark Albion is unlocked once you have completed the credits level of the main game. Dark Credits is the seventh and final level of this second playthrough.
As you play through the level make sure the attack the multi-coloured developer names which adorn the back of the screen. I found this easiest with Reaver's pistol. Smash 20 (I destroyed all of them to make sure/for the coins). You hear a cymbol chime when one is destroyed, and it will disappear from the screen.
There are a total of 64 in the level I believe, so sadly in a 4 player game you can't all get it.
Once you complete the level (by beating the two bosses) the achievement will unlock.
This can be done in single player with AI team-mates.
If you're struggling Maka91 has made a fantastic guide for this achievement:
(All credit for video to Maka91)