Note: The information here is from when there was a Level Restriction at 50. Any job at 75+, and even considerably lower, can solo this mission with varying difficulty/speed (see testimonials). As with other Promyvion areas, there are aggressive monsters around level 80, but they are easily identified from their names and can be avoided as long as players pay attention to them.
Promyvion - Vahzl is five islands instead of four, and the monsters are stronger than in other areas of Promyvion. Like the other Promyvions, it's only tough if you or your party is aggroing everything in sight. At level 75, although many monsters will check as Easy Prey, nothing will be a high-enough level to aggro.
On the third, fourth, and fifth islands, you will have to search for Memory Flux gates before proceeding to the Memory Receptacles. Each will spawn a Notorious Monster, and these are mandatory fights that are required to proceed with the mission. If you do not defeat all the three Memory Flux NMs and get the cutscenes at the Memory Fluxes, you will not be able to cross to the Web of Recollections within the Spire of Vahzl.
Make sure to examine the Memory Flux after defeating each NM.
Fourth island: Solicitor (Craver type) has a dangerous single-target attack and an AoE Poison attack that deals damage and gives a strong poison effect. It is advisable that jobs that cannot cure poison have a few antidotes. Also, if needed, have someone cast Gravity on the NM so healers can Cure without worrying about any threats.
Fifth island: Ponderer (Thinker type with AOE Sleep so bring poison potions). The Ponderer can also absorb food and spell effects, so make sure your tank doesn't eat defense food when you're getting close to Ponderer. If it absorbs a Tavnazian Taco and Protect it is difficult to hurt with melee attacks.
When you receive the cutscene at the Memory Flux, you gain the ability to warp directly to that Memory Flux from the gate in Pso'Xja. This allows you to bypass the lower floors in case you need to leave or are helping someone else with the mission later on.
The NM at each Memory Flux will re-spawn if a player leaves Promyvion - Vahzl and returns, then clicks the Memory Flux again (while the mission is still active). This is not necessary and your party can wipe easily this way, because the NM will aggro any player who teleports inside. In the rare event that this does happen, the NM will de-spawn if left unclaimed for 3 minutes.
Spire of Vahzl
Buffs will wear upon entry.
Enter the Web of Recollections and be prepared to face three bosses.
You can choose which boss to fight first. One of them will join the fight when the one being fought has about 20% of HP left. The next boss will not attack if you kill the current boss you are fighting in one shot from above 20%. For example: weapon skill at 25% to kill it and the next boss will not attack.
The only other way to get them to attack is to attack them first.