Easy once you know where is the key.
First you have to dig those 4 light boulders, even if you got a saw it cost less energy to destroy them than the cooper blocks. Dig the dirts to make the first medium boulder fall but DON'T release the two others: they'll block the door if you do so. Just kill the blob with your knife, open the door, kill the second blob and take the key.
Now you can let the two other medium boulder fall, don't stay under it (medium boulders hurts a lot). Chop the sticks, kill the last blob and the treasure is yours.
You can get the plank and the emerald without special tools but you can't get the crystal without a hammer. Also be carefull about the medium boulder above the sticks and the other one above the soft stone block, move fast if you don't want to get smashed.
You'll also need a saw and a green key to clear that mine.