Priest has three checkpoints to hit. Level 29, when he gains Master Alchemy. Level, what, 36 or something? - when he gains access to 3rd level spells (including Exile, which would allow him to oneshot undeads and demons - use of Bless is mandatory with it; also, Divine Mercy + Holy Ground, which cuts the backtracking in half). And level 45 or something - when he gains access to 4th level spells (including Transsubstation, which allows him to oneshot anyone, although at a cost of a longer cooldown - the use of Bless and Archangel's Word buffs is mandatory with it).
The hardest thing is to get to 29th level alive (actually, if you don't intend to grind like mad, you'll hit it after Act1, so that the entire orkish dungeon needs to be completed without it). The best bet there, I think, is to use Azrael-prefix bow (double the damage; farm the blacksmith in the first town until you get at least two of those - with any suffixes whatsoever; you won't need a lot of money for that, 1500 ought to be enough for both combined, but you do need a lot of time - it's advisable to target Azrael's War Bows, because those are the best bows priest can use, but actually any Azrael's Bow period - except composite ones - would be good) with some +12 arrows (no reason to use the ordinary ones, the price on +12 is only very slightly higher) and that fire damage buff from the Miracle Magic, Blade of Fire. Plus the Open Wounds spell from Inquisitorial Magic (and Crucifixion from there as well, for those monsters which are Just Too Quick for you, for now). Oh, yes, when playing as an archer, use keyboard ONLY for moving, remap the controls from cursor keys to WASD using Auto-Hotkey tool, and remap what WAS on WASD to somewhere else (since you won't be using mouse for navigation, when in dungeons and not in combat, use it to check the walls, while going down the corridors, the "hand" cursor will appear even on a "non-colored" - not noticed by the character - secret, also use Sixth Sense and that other spell from Miracle magic, something connected to eye or vision, to buff the Intuition, and always check all the stones and stumps even if they are not "colored"). Collect and sell everything. Use genie boxes for skill points only (after destroying the box, choose "I don't need anything", then see in log, whether "+1 skill points" appeared, if not, reload dialogue autosave). Use extra money (after repairing and restocking on arrows and potions) on genie boxes only. All the items in Inquisitor are generated in terms of their stats, even unique ones. Their stats are either generated when you first enter the location they are in, or when you receive them over the course of a dialogue (which happens rarely). The latter case can obviously be exploited.
When you hit 29th level, everything needs to be in place (stats, skills) so that you'll immediately gain Master Alchemy (also, the items that "increase skills" don't count towards gaining access to "perks"/skill_levels, so you NEED to have 16 alchemy fair and square by that point, not counting any items). Having Master Alchemy allows you to do mutator elixirs (+1 to chosen stat) without devil's root and without paying - how much? 1000 or something? - to healer; from 4 miraculous potions (which cost only 100+ for all 4 needed for a single elixir), put it this way, you can buy a single genie box, or you can make 35 elixirs. Anyway, from that point on, every single gold piece should go into the making of the elixirs. Stats are not capped, so that the only criterion to stop increasing them would be "eh, can't be bothered anymore". I recommend increasing speed until you definitely stop noticing any visual difference in the actual speed of movement of your character (so that the monsters can't overrun you), then increasing Dexterity and Intellect as high as you care about. Dexterity is connected to the armor rating - and it's directly connected to your bow damage. Every 10 points of dexterity give you +1 to bow damage, Azrael's bows make that +2, on 1000 dexterity you would have +200 to bow damage. Spells do NOT have this kind of direct influence of stats on their damage, so, until you get to oneshotters (Exile and Transsubstation) it's imperative to make as much use of the direct damage as possible. And yes, even with 1000 dexterity and the corresponding armor rating, you will still get hit, surprisingly frequently so, actually. Intelligence has direct proportional link to magic resistance, but it's actually possible to complete the game without overclocking it (I didn't overclock it, and completed the game). Oh, by the way, the Locust Swarm spell goes past all the resistances (just like spikes from Divine Divinity), and one can spam it like mad, so, it'll have its occasional uses even near the end of the game. From this point onwards, the game becomes as piss easy, as you put your back into the "overclocking". Or rather, it becomes _just_easy, the piss easy part is after you get oneshotters.
When you are content with the stats, start, again, spending all the extra money on genie boxes, and get Divine (because Bless and Transsubstation), True Faith (Exile) and Miracle magic (Locust Swarm, Archangel's Word), as well as Wisdom to 20 as fast as possible. With that, you'll oneshot literally everyone, without any grinding, just as you encounter them, save for, like, 5 monsters in the entire game (and that's when you'll need Locust Swarm), you'll also start to seriously like all the undead and demons because Exiling them is thrice quicker than Transsubstating everyone else. And yes, now it's officially piss easy.
Also, you can totally empty all the private chests even if you do not play as a thief. You need to place every single item from your inventory on the ground (nothing will happen to them) and spend all your money on whatever, leave only one bow and a couple of quivers with you (you can also use Shatter spell; I didn't, because Pagan Magic; hypocritical - seeing that I'm talking about thieving right now - I know). For each act, you need to do all the chests at once. Go to the chosen chest, break it, immediately (before the spawned warder gets to you) throw the bow and the quivers away (it IS possible, experiment on that, also, the game can be paused), choose "I surrender everything I carry" (which is nothing), do not take anything from the chest, pick up your bow and quivers from the ground, go to another chest, rinse and repeat until every guarded chest is opened. Go back to your stuff and collect it, revisit every chest you've broken into and grab everything inside of it, sell everything you don't need, use money for more elixirs and genie boxes.
P.S. In Act3, it's advisable to first clear the locations with the undead (using, naturally, Exile) and finish whatever quests you can, so that when once you begin working on werewolves, you'll already have 4th level spells, Transsubstation included. Come to think of it, the same applies to the undead in Act2. Clear everything (using your overclocked bow damage and armor rating) non-undead until you gain access to 3rd level spells, Exile included, then do the undead.