This Pso'Xja tower is uncapped. Follow the path forward until you find a stone door, which, when checked, spawns an NMGolem named Nunyunuwi.
This NM has approximately 3500 HP, though offset by an innate Auto Regen trait that restores 100 HP per tick. If improperly handled, this effect can draw the battle out indefinitely. As a result, Nunyunuwi is often defeated with as much damage in as short a period as possible. Among the methods known (or theorized to be) effective are:
Use of high-level SMNBlood Pacts (when melee or additional SMN support is available)
This NM is known soloable by some jobs at, or over, Level 74 (RNG, MNK, BLM, RDM, DRG, DRK, SAM). Soloing at slightly lower levels may be possible.
When Nunyunuwi is defeated, check the door to move past it.
It is advisable to cast Sneak or use a Silent Oil then head down the elevator.
Check the Stone Door for a cutscene. You will arrive in Promyvion - Vahzl, where you will get another cutscene.
Game Description
Mission Orders
The airship piloted by Louverance did not arrive in San d'Oria, but Port Bastok. How much longer will Nag'molada wait for your arrival at the northwestern tower of Pso'Xja?