Fireflyin's Shanty Guide to Spirit Tower.
What I seemed to collect from this dungeon, is that the only requirement is to be level 65. This dungeon reminds me a lot of facing the event boss Yoh, as it appears attack and damage taken all works off percents.
So the first two skills both seem to take down 3%, together 6%. so you have to hit the boss around 30 times to take him down. These skills have a 2 second cool-down, and should be spammed continuously. I find what works best is to use the 2 key skill first followed by the 1 key skill. This allows better evasion as you can move sooner after skill 1.
Keep in mind that skill 1 is ranged, and does not have much skill casting time, while the 2 skill is AoE around you (10m) and has a long animation and "can't move" time shortly after.
The 3 skill is the stun skill, and does the most damage at 5%. It also has a small cast time bar, like the heal spell. This skill should be spammed immediately when boss spawns, followed by continuous 2 and 1 spam. This skill is especially ideal for when you "cannot avoid" incoming red, and need to stun boss to move away, so keep in mind the cool down, but still try to use as much as you can!
As for the 4 skill, healing. I try to use every time health is lost. Also be sure to use in between bosses when needed. This skill is kind of like the monk's tiger claw with it's ready to cast bar, and you cannot move for a half second or so after clicking the skill. Always heal before dealing out more damage, because revive time is much more costing than getting a couple more hits in.
The spheres. From what I've noticed, the big blue ones without any text just make you move from "incredibly slow" 100% to "super ms hunter fast" 200%. The smaller orbs have a boss text thing, and should be avoided at all costs! hitting one will summon a miniature version that is just as tough and annoying as the one you're facing- maybe more. So keep your eyes peeled and don't run in to one. They can spawn anywhere! I think they can even spawn right where you are! So rip, if that happens.
Lastly, being that this is a % dungeon, meaning your character/class stats don't matter. Mechanics is everything. Getting to know the bosses helps considerable, knowing what attacks hurt you little, and what attacks hurt you a lot. Keep mobile around the boss spamming 2 then 1, and 3 when you can. It seems like every boss has a similar pattern, kind of like event Yoh, where they will always have a few second pause before each big "avoid the red" attack. So try to time the 2 and 1 skill with their incoming red attacks for best evasion / lots of attacks getting in.
This is a dungeon where your stat's don't matter. Only your level of knowing how to attack/evade red/ and heal when needed. It's frustrating because we don't have our stats to hide behind, but also challenging and very rewarding - good luck! it is VERY possible to complete, just takes a little practice! I hope this helps! Have a good night!