The Masks and High Spirits have several things in common - High damage, high defense, ultra low HP, and somewhat long turn number.
(3 to 8, and the only exception is the Mystic Mask which attacks every turn)
Demon Masks:
15 HP
Divine Masks:
18 HP
Mystic Mask:
20 HP
Double Spirits:
10 HP
Double Rainbow Spirit:
15 HP
Now, because their defense is so high, we will be doing 1 damage only, but that's all we needed. If you have an all water team, by just clearing a single water combo, you will hit the enemy 6 times, taking out 6 HP, so in just 3 to 4 turns, you can kill them no problem!
When you entered the stage and you saw enemies with different turns, congratulation, you have it easy now, since it will take a really long time for them to have the same turn to attack you at once, you can take your time killing them one by one.
But what if you met multiple enemies with the same turn numbers? You don't have to worry that much. First of all, you usually have 3 turns or more to carry out your attack. Send out your AoE attack several times in a row and you should kill them like a piece of cake.
In situation like the picture above, there are two Divine Mask with the same turn number, you just have to take out one first, then deal with the last 3 later. With the Resolve skill, unless two of them eventually got to the same turns, you won't be killed at all.
It is important to note that: You only need 1 combo
(of the element type you're using) per turns, doing 3 combo of water orb will still do 1 damage, so you will just be wasting your water orbs for next turn. Try to eliminate as many other useless orbs at the same time too.
It is also a good idea to store orb
(or active skills for some setup like poison) every time before you enter the next floor, just in case.
There are several way to store orb, one of the example can be seen from my previous picture, putting 4 orbs to the corner, and do:
(somehow my images' edge got cut off a bit, sorry for that)
It's easy, and you can do it for both sides without affecting each other that easily; just remember to heal yourself up when you're storing orbs.
Because this process can get repetitive, sometimes you might accidentally forgot to heal before the enemies attacked. So it's a good habit to
1) heal yourself back up immediately after enemies just attacked you, or
2) even when you don't need the auto-heal from Siren, always do orb elimination every turn so you don't forget.
Once you get to the final floor, just watch out if there are any Divine Masks with the same turn number, use orbs you stored for AoE attack
(or just use poison if you have the skill up, instant KO).
And you're done! The same can be done for the Thursday Dungeons, only difference is that it has the small Spirits for the first 3 floors, and they're pretty fragile, so be careful when you're storing orb so you don't accidentally kill them.
It is entirely possible for you to meet pure Double Spirits setup in floor 2 and 3 in a row. If multiple of them somehow got the same turn number and you didn't store orbs, good luck to you...-v-
That's it for the guide. This setup is not a 100% clear method, nor does it only require brainless effort, but it's definitely a big help to those who don't have the strong or poison monsters. Good luck hunting your material ^_^