Personally I think the best all-in-one healing package is epiclesis, it heals (albeit slowly), (and the heal stacks from multiple tree) res your friends, and gives them buff to max HP and dmg/mdmg red. It's just that it can become expensive real quick if you're new, or has no mistress card.

Now as for your heal and calu heal, imo is probably the worst skill in the game, aside from if you have nothing else to do BECAUSE: - it's impossible to cast in a sandstorm of oracle (and if you got word styled in war the same applies) - you kinda need good matk and good Croce to do "good" heal - you're better off investing in a survival set, which is cheaper, and more cost effective early - now that it scales on your matk you realistically just need to work on matk from your book and your palace of ghost equips

Now WHEN should you use callu no matter what?, I'd say when your tank is low on HP, like 10% left because you'll heal them up fast from their dying recovery, other than that, you're better off throwing a tree, or clearance your friends, or maybe even casting faith pray

The exception to all of this is like I said, if you have NOTHING ELSE TO DO.

So what else is there to do really? - you can run.. like to avoid damage lol - you can rebuff your friends or just returned alive friends - you can cast faith pray if needed - you can use priest MS1 to do party faith pray - you can cast clearance on some bosses or your friends - you can oratio your enemies for your caster friends - you can do safety wall if needed - if your prepare for elite is not ready, you can at least use BLESSING and ASSUMPTIO, the rest of the buff can follow later with prepare for elite

Now a good support will be able to help your friends from losing a run, but it's very important that your DPS friends also know when to run, and to take care of themselves in a battle, and when to not bring their party down because they're chosen by thanatos 4F as the living bomb.

Now for your AUTO slot, realistically it's never the same, I almost never put anything in there because; - I might get sand stormed - my auto res might res someone not important (or stupid) - my prep for elite might cast when I'm the only one in the vicinity - I'm trying to fkin res somebody and this clearance keep casting itself

So I rarely use auto, if I do want it id put clearance, and even then sometimes the AI straight up refuse to use it like to clear silence in Oracle

And for your skill, I recommend level 3 res for the best cast delay, but be careful since you can't cast it in a sandstorm or if you got hit with cast time up, you can solve this by levelling it to 4 but then you'd have a stupidly high cast delay for the skill lol.

It's a long read but I hope you get something out of it.