Is there a video-guide showing all treasures for it?
I just used
this map here. There may be a video guide around, but I'm not aware of it. I'm not generally a fan of constantly following specific instructions like zone here, activate this, zone there do this. I just used the map to show me where everything was and then, figured out the path on my own.
You'll start in Area 1, so just get the treasures before you zone to the next Area and so on. If Area 3 looks confusing, here's a rough guide. You'll start on waypoint XVI. Unlock the top left Libra (Orange) gate on the same platform as the Virgo (Blue), go to the Capricorn (yellow) switch ane use it to run to and unlock the Capricorn gate next to the Virgo switch. It doesn't matter which Virgo gate you unlock, just pick whatever though the top left one next to Libra will give you more time for Taurus later) After you've done the Virgo gate, go back to the Capricorn switch and unlock the other yellow gate on the same platform as the blue one above. The only Aquarius (white) gate you need to unlock is the one on the left next to Taurus (pink/purple) switch, and for that, unlock the gate leading to Hastega first, and then the other one leading to another treasure and Omega XII.
It's actually a lot easier and less complicated than it looks and sounds.
Also if you have a Black Mage, I strongly recommend getting Scathe (Near the 2nd Gilgamesh fight in Lhusu) since it works wonders at clearing the hordes of enemies you'll come across therez even better if you keep them on Faith and full HP. If you have WHM or MNK that can use Holy (Pharos at Ridorana) even better. Just pair said Holy ueer with White Robes for extra damage (also in Pharos).
When I was in the 2nd area of The Great Crystal, Penelo (BLM/MNK) just wrecked everything pretty much by herself. Balthier (WHM) and Ashe (KNI/TIM) just kept her buffed, alive and status free so Penelo could Nuke everything. Balthier chipped in with the odd Holy, and once Ultima was down Ashe came into her own with Excalibur as well, but Scathe is just too good if you have a Gambit to use it on 2+ Foes.