Probably mid-way through the Dragon Lair stage (near the end of the game) you'll get on a mine cart, and you'll pass by a bunch of soldiers in stone. You can free them by attacking them. You need to free a bunch for the achievement. It seems they changed the requirements because getting them all is nigh impossible due to having to recharge mana so much, and from what I can see reading around it is now 18. The easiest way to do this is probably with the ice mage, because his regular attack is a spread, meaning you can hit groups of the soldiers at the same time. I didn't try it, but his blizzard spell
might be great for this if you time it right, he swings an icicle around that might hit the soldiers. I got it first try without even knowing I was going for it though, so it shouldn't be too hard if you've been upgrading his ice bolt (the regular attack).