First off let me warn you that i use even my heal tiles to dps, but yes, the residual tiles i use for healing, be it myself or my partner.
As for filler rank if you refer to Krivis, i only took due to the lack of better class, i could go cleric C3, but it would honestly be more of a waste than anything. At this time i wish Sadhu was unlocked one rank lower lol.
At lower Lvs being a Priest could help on killing flying enemies using blessing, but when you hit Lv 75, you will notice that the damage is not even that good, and you will expend a good 30 seconds to kill something only with basic attacks, not to count that you will take heavy damage depending on the monsters.
Indeed having resurrection would be interesting, but our function in the end is DPS not support, and i am sure there are better builds that would complement those classes better than Sadhu and Druid.
Zaibas is not really that much of a skill tbh, but at least it help kill some stuff faster, specially if they are ice element mob, then Zaibas shine in damage output, but on the counterpart, it is deception against earth element mob, the damage drop by half from normally.
But still, it is a good filler until you get Sadhu, to cover a bit of that you can use SZ/Fade + OOB to kill the flying stuff that come close to you.
Now as for the other skills, it depend on the monster types, dark element mobs die fast as you know, since most of our skills are holy damage, while positioning, which for me is extreme difficult since my latency is always over 300ms (Brazil here), i generally try to move for a while leaving the monsters at least 10 steps behind me, then i turn around, and since they are in line trying to catch me, i turn around and activate possession, this allow me to lock them all with safety.
Note that the above i only do when my Fade or my SZ is on CD, even so, if i try to only move a bit, use SZ and cast possession, i have issues losing 2 or 3 mobs in the AoE since my high latency cause the monsters thing that i move more than i really did.
So instead of waste an important defense skill on a lure that i know it will have a 20-30% miss, i save it for Elites with high HP pool and heavy damage or magic or archer type mob that cause heavy damage.
Cure is a good skill to any time, cast it, lure the mob to it, if the mob go around position and jump over it to position the mob on top of it.
My experience at Demon Prison was a breeze since most of the monsters there are dark and demon types, so your holy skills will shine there.
Unfortunately i didn’t had the luck chance to find Harpeia there, so i can’t tell how well it would be against a WB.
I am still going to Druid C1, so i can’t say how carnivore will fare for me, if i does the same effect as possession (stun) even tho only work on ground mob, it still a + for me, i can leave it there doing damage while i can cast possession on the flying ones.
As for gear, i think it depends on how much at easy you feel while luring and killing, i am using plate pants and boots, and i still am using my Rokas Robe (the purple one from the quest) and a mage gloves Lv 120, my neck is that Electus, it help a little bit on Zaibas, but not a miracle lol.
As for my weapons, i am using these with awakening:
It do increase well the damage, specially that 50 holy damage on Valias.
My stats are a bit extreme, but it is the way i feel more at easy:

When i hit 100 CON i will put the rest in INT.
I still need to get some good gear, i might pass to all cloth later and increase well the armor attribute.
Sorry for the text wall, but that is my experience so far, i am by no means perfect playing, specially with a messed up controller mode and high latency as i have here, not to count the stupid stuttering i get sometimes.
But i am having fun with this build, i love caster type char, and this one is giving me what i need so far.