Either a flat wrecking bar or a wrecking chisel.
Make sure you have a high quality flat & wide wrecking bar. And i cannot stress the word "high quality" enough.
Method:
1. Insert it near a corner with the short end, pry slightly
2. Insert the long end in the slot you just created near the corner.
3. Slide it right down to the other corner.
4. Pull away from the wall from the now released edge.
5. ???????
6. Profit.
If you use a wrecking chisel use a bloody rubber hammer to avoid massive recoil blowing your wrists and to dampen the sound.
Method:
1. Hold the chisel near a corner.
2. Slam on it with the force of a thousand gods.
3. Repeat.
4. Pull away from the wall from the now released edge.
The tools of the trade: