Mages do not suck, but the more effective and mana efficient spells only start to become available in act 2 (particularly Shaman magic).

The DeathKnight gains armour based on level, so doesn't need strength for armour requirements. Built as a mage or bow archer, this would allow more focus on intelligence or agility, etc.

An archer as a non-lead character, set to agressive, will auto-attack opponents in range, which makes a nice early warning system (ie if you have the inventory open checking out loot you just got, and opponents just off-screen approach).
The act 1 summoning doll can equip a crossbow, so can provide good support for tough fights. Even if upgraded summing dolls are not that tough, but it would do fine if kept out of melee combat (and at least provide a bit of a distraction).

I put a only couple skill points into weapon damage and accuracy, but maxed Reload Time for my archer (gives a direct Speed boost).

At the beginning of the game the DK has a sword and the hero can get a club, which works well given the number of skeletons at the start of the game. I kept going that way most of the act, switching the DK to 2 handed swords and my hero to a two handed walking stick after awhile (and sometimes using a bow for tough fights). Once I reached the citadel near the end of act 1, I switched my hero to be an archer full time. It would have been more optimal to make the DK a bow archer, but I still did fine.

BTW I collected special arrows, but rarely used any (they were rarer in the original release, and the BF merchant didn't stock them regularly). I mostly used normal arrows (playing again, I'd switch to power arrows much sooner) and splitting arrows came in handy in a couple places (particularly if you can set up a crossfire situation with the act 1 doll, against a group in a confined space). The elemental arrows, etc, would be much more effective against opponents vulnerable to that type of damage, but I rarely found myself in a situation where I needed the extra damage enough to bother swapping arrows around (though you can set hotkeys to select from 3 different arrow slots).


Beyond Divinity Strategy Guide

The walkthrough section contains tips throughout, and of course spoilers. There is a math appendix that gives info (and advice on stat distribution, among other things), plus sections on skills, creatures, equipment, etc.

There is some initial stat distribution advice in the topic Killing Fergus.