I was doing this quest so I thought I'd record it in case anyone needs a quick walkthrough of the whole thing on the latest patch - 1.2.1.2.
Video is just over 6 minutes with most footage at x2 speed.
Rewards are an attribute point and a skill point, plus the three Guardian bosses drop unique trousers/pants which are decent depending on your build needs.
Exploit - you can stockpile the runestones in your personal stash, meaning you can collect multiples of each one, then go into Ultimate, drop them and pick them up to trigger the quest and complete the first part which will give you TWO attribute points because you haven't done the quest in Elite yet. Stockpile them BEFORE using them to open the final stone door because this will delete them from your inventory and prevent you collecting more in that difficulty.