It seems to me, that casters should prioritize casting first, with a secondary focus on physical damage (note that this game can't be completed with magic damage alone, as there are bosses).

What is almost required for a full caster is to max Intelligence, Speed, and Senses. In this way, you unlock PowerCast, SnakeSpeed, and Eagle Eye (good for your slings on AP), as well as getting maximum Initiative.

The other stats can be argued until kingdom come: Strength, Piety, Vitality, Dexterity.

Battle-Priests (probably the only viable combat caster), should go for: Strength, Dexterity, Speed, Senses (same as a non-caster).

If going for 4 stats, I would say, go with a traditional caster build: Intelligence, Piety, Speed, Senses (casters' hit points are always going to be terrible).

If going for 5 stats (my Human Psionic will do this, by level 36): I'm going for Intelligence, Piety, Vitality (hit points sorely needed), Speed, and Senses. The dump stats of Strength and Dexterity can easily be argued as the least important, as how often do you think he will be swinging his Staff of Doom?

I sometimes vary things.

It's typically a good idea for a character to have at least 2 major specializations. as there's way too many pints and practice opportunities for a character to have just 1. With a specialist caster, one of them will be the spellbook skill, but you're going to want another one.

Here's some of what I would consider a major specialization:
* Close Combat (+ weapon skill)
* Ranged Combat (+ weapon skill)
* One of the 4 spellbook skills
* Music or Engineering

So, I would suggest choosing at least 2 of them and focusing on those skills with both level up stat points and skill points/practice. Doing more is possible, but starts to dilute the character unless you really like skill practice (like I do with magic skills, hence why I like Bishops), or you expect to reach high levels (30+, likely with small parties or if you're RChu1982).

Single specializations are, IMO, only good for characters you plan to replace entirely.

Tuning casters into fighters, especially to fight end-game battles, is mostly futile when it come to reaping results.

Under casters I understand: Wizard / Psionic / Alchemist

Well, Alchemist sometimes can deal some damage, but don't expect him/her to tip the scales of end-combat results.

Instead, I would train them in throwing (which is easy to do, as it also entails sling & stone weapons, so it can be done while fighting weaker enemies during common farming) and artifacts.

With Throwing + Artifacts they can use scrolls, bombs and such, that might have effect, where their own magic fails.

unmodded W8 offers very little in the meaning of melee or ranged weapons for the the three (Priest can have some so-so weapons and even armor, but fails in ranged combat equally miserably)

Even if one would have Wrist Rocket + Medusa Stones -> stat-wise will all casters remain very weak, so either not hitting their targets or rarely doing damage even if they hit.

Also, hoping that weak (physically) classes might do good job in front rows brings often a big disappointment in form of quick demise of such bold melee casters. Even if enemies might not pick on them (and to my experience, they do), even random hit from mid...late game enemies can be simply put the typical one-hit kill, but it will be your char dying.

While there is nothing wrong letting your caster train ranged or close combat against mediocre enemies, posing little threat. I would advise to abstain from spending valuable skill points during level-ups on said skills. (with exception throwing & sling)... maybe after you got all magic schools >75 (and always invest max point in power cast)

While it is very true, that eventually casters will become a lot weaker against highly resistant enemies, turning them into melee fighters is but a dream.
Just be happy how they helped you progress in first 60~75% of the game, speeding not only fights but also travel, being often very versatile.
Remember, the game can be finished with one character only, so having 2~4 fighers should be enough to tackle highly resistant enemies.
It is ok to let sometimes some of your characters sit it out on substitute bench from time to time. Just like fighters often do nothing, while enemies are wiped out with fireballs or mind flay.

And as mentioned - from the variety of usable objects, often you can find something that even a spellcaster can use/throw... giving him extra value (or at least >0 value) in a difficult combat.

I also feel frustrated from pure-spellcasters dropping combat efficiency, but I still go for them due their skill 25% bonus, that gives them extra edge for most of the time.
Otherwise, one could rather go for hybrids, who are indeed deadly in melee or ranged combat, and keep max 1 pure spellcaster or even none at all. I tried such playthroughts, and it didn't always work out for me as I expected, but is still feasible. (if I may to share an advice: keep at least one psionic, to save you from mind-altering enemies: to tackle down status effects like turn coat or insanity... )

The weapon options for a priest late-game aren't just so-so; they're actually really good.
* Diamond Eyes, if nobody else is using it, is strong enough to be a good main hand weapon. Also, Paralysis from the weapon can work well on water-weak enemies like Rapax.
* The Mauler is actually stronger, but slower. Since it's buyable, you can get as many as you need. I don't think there's any stronger weapon that's buyable.
* A good sling with good ammo can actually be good. One could also consider the Doubleshot Sling to get an extra swing with each attack, and with Eagle Eye, the accuracy penalty isn't a big deal.
* There's no reason you can't build a Priest's stats to make them actually good at combat. Stats, particularly at high levels, are not tied to class.
* The only significant weaknesses of the Priest compared to hybrids are HP and attack rating (which contributes to extra attacks). Not when we're talking enemies like El Dorado and that one enemy you fight in the Cosmic Circle for 2 of the endings. In those cases, you're better off just bashing them in melee, even if a caster. In my experience, this isn't a problem for Priests, except maybe if the enemy in question is *really* strong, and even then, there's Guardian Angel and Body of Stone.

The Human Bishop I had in the front line did have some issues here, but perhaps using a Dwarf instead might solve it.

Also, for late game uses, there's that one cloak that adds +20 Stealth, so that the enemies will be more likely to attack your other front line characters instead. (Just don't put the character alone in the front or pair them with characters who naturally get Stealth.) Furthermore, by that point there's a good chance multiple characters will know Resurrection, particularly if you're using a party that's magic heavy enough to need to put full casters in the front.

Don't forget that you could choose to go for Reflextion or Iron Skin for more protection. I disagree, at least for specialist casters. Your magic skills will increase nicely enough through use, particularly at mid-high levels when you can try to cast higher level spells at higher power levels, but your fighting skills won't be as easy to boost before you get that second attack and swing (which comes late for full casters).

There is no debate with 3 of the specialist casters: Alchemist, Psionic, and Mage want Powercast, and should equip Staves of Doom (farmable from Rapax Corpses), and Wrist Rockets with Medusa Stones. This is because they have poor one-handed weapons. Note that the Psionic and Mage have terrible hit points and armor, and can't use shields at all. The Alchemist, while able to wear decent armor and equip shields, is still limited to terrible one-handed weapons (daggers, wands, short staves), and should not use said weapons with a shield.

The Battle-Priest that I am creating (can use Mace and Flail weapons with a shield), is maxing Dexterity and Speed first (early Reflextion and SnakeSpeed), and will max Strength, Vitality, and Senses by level 41 (dump stats of Intelligence and Piety even, at 60). He will use either Diamond Eyes or The Mauler, whichever gives more average damage, with a *Light Shield*, dropped by Jan-Ette, after talking to her enough times. Obviously, Wrist Rocket and Medusa Stones for range.

Non-casters (Fighter, Rogue, Bard, and Gadgeteer): The choice is already made. Intelligence and Piety are very clearly the dump stats (they get no benefit from Powercast, while they get no spell points from Piety). Strength, Vitality, Dexterity, Speed, and Senses.

Those two things don't mix. If you have a Staff of Doom, you can't unequip it without the help of a Remove Curse spell or a high level Bishop (and the Bishop's level needs to be pretty high in this case).

There's other staves you could use instead.

Also, if you have a Psionic and go for the Mindblast Rod, you might want the Psionic to be the one to use that item. If your Alchemist isn't making much use of melee weapons, you might want to equip a shield anyway, since there's nothing preventing you from casting spells or throwing bombs with one equipped.

Also, don't forget that:
* The Winterwand can paralyze. Even if you're not doing much damage, if you can get the paralyze effect to land, that's still quite helpful.
* The Canezou Dagger is 3-14, which isn't bad for a dagger and can be used by anyone (yes, even Priests and Bishops who lack the Dagger skill).