This is a perfect congruent conflation. It mixes “getting blood out of (or from) a stone” and “like pulling teeth”, both phrases meaning to do something with great difficulty. The speaker was finding a particular essay difficult to write and remarked that writing it was like pulling blood out of a stone. A big thanks to Red C. for sending this one in from the U.K.
This is a mash up of “locked in” and “written (or carved) in stone”, both meaning something permanent or not subject to change.