Now, the wiki has everything, really.
This is the uncleared map of that floor. You see where the stones were at the beginning:
AND you can come from two sides to that part. The whole location can be done in a circle.
So you move around the two stones and pull the left stone as far to the left as possible.
Then back to the reward area (through exit 7 and taking the boat) and on you go through all floors to where you are standing now (through entry number 6).
THEN you can go around the remaining stone and clear the last tiles.
If the boat is on the wrong side:
Move back to the reward area through 6.
Cross the lake with the boat, go on and through 7 to this place. Pull the left stone as far to the left as possible.
Then back to the reward area with the boat, go round and come through 6, around the remaining stone and clear the tiles.
