Go back to the obsidian mine. Simply click on one of the striped areas of wall to the East of TzHaar-Hur-Brekt to obtain a piece of stone slab. Spam clicking upon each wall will increase your successful mining rate drastically. Two pieces of stone slab can be mined from each 'wall' before the ore runs out (40 Mining experience each), and the re-spawn rate is quite fast. Gather 20 stone slabs. Create some stone pillars by using the left click option on the slabs; they reward 20 Crafting experience each. Spam clicking the last stone slab in your inventory will result in nearly all of them being crafted at the same time. You will need the pillars you've created to repair the tunnels.