If you want a full support that can do damage in solo quests, this route is actually viable which I am currently play with the build of high SPR and decent INT.
Cleric>Krivis>Priest>Priest>Cleric>Priest>Plague Doctor
But in this build, I only got Heal (10), Cure (1) and Zaibas (4). The AA is not bad with Blessing and Sacrament. I considered the damage I did decent but it is slow. Most of damage is done by Zaibas since it scale with INT which is not bad. Also, Zalciai’s magic amplification help me boost a bit all the magic skills I have.
And if you are willing to sacrifice 1 skill point in Priest C3, you can always get Exorcise (1). Just 1 level is enough while waiting for other CDs. And when you get to Plague Doctor, at least have Incineration (1).
Unless you are full support with full SPR, it is not hard but really really slow to solo quests unless you get a party.
Of course, this is simply from my little experience and opinion regarding damaging at solo quests specifically with quest’s bosses. 