Yeah, I've done a couple of these. How stuck is it exactly? Have you released the clips or is it still 'all the way on'.
Lets start with the simplest possibility:Have you missed a screw somewhere? Did you get the one on the left under the foil tape? (You can put the tape back afterwards, it stays pretty sticky.)
The power and speaker cable are on the left side, they will stop you removing the case entirely if you have released the clips.
If the clips are still stuck, get an old credit card (not a valid one - you will likely destroy it) or preferable something slightly stiffer (though no thicker), and see if you can squeeze it between the top and bottom cases. Its a matter of working your way around the case from a point where you can get the card in. The card should be inserted 'pointing upwards' so it goes under the top case, rather than the bottom. DO NOT USE ANYTHING MADE OF METAL to release the clips, unless you want to SCRATCH THE S**T out of your case. And make small dents in the edges.
If you're still stuck, try googling "apple service manuals" and maybe you'll find the one for the 12". Theres actually a more than one for the 12", but they're not massively different. It won't tell you much more than I have though. But perhaps the pictures will help.