Managing a team
I think I might as well take some initiative and put up a decent walkthrough around here.
Your team is very important. Although it is possible to have a team composed of only one class (best done with cleric and warrior if you're interested), it's much better to use a more well balanced team. That means that you not only need to have a class that can hit hard, you also need to have a class that can keep your hard-hitters alive. Attack types also need to be taken into consideration. Most fights will involve both the front and back rows, and you need to be able to have people that can hit enemies in the back.
My personal favorite team is a warrior, cleric, mage, and ranger. The ranger and mage provide ranged support, while the warrior and cleric are on the front for close combat. While warriors and mages can hit very hard, the cleric and ranger heal spells are very useful for keeping them alive and for removing debuffs. I am not very experienced with rogues, and even less so with barbarians and conjurors, so I won't discuss them.
Rangers: Rangers are a pretty interesting bunch. They're most effective in the back because of their medium armor, although they can be outfitted with melee weapons. I prefer regular bows over crossbows most of the time, because crossbows wreak havoc with your quickness. Rangers are one of the quicker classes, so use them to heavily weaken key enemies. The loadout I like to use is Ranged Attack, Pierce, Nature's Balm, Healing Lore, Envenomed Arrow, and Focus, with passive points on Vitality. The ranger's other skills aren't too bad (Stun in particular is a very viable alternative for Envenomed Arrow) compared to the skills other classes get to choose from. When battle, I usually like to start with Pierce to wound two enemies at the same time, then move into a support role. Poison damage on Rangers is a very powerful choice. Use your Orbs of Toxicity on your bows whenever you can.
Mages: The mage is your standard magic conjuror. Because many units like Voidstalkers, Aetherguards, and Wraiths are resistant to physical damage, a mage can do wonders against them with their magic attacks. Although mages can wield swords, putting them on the front line is a very poor choice because of their light armor. Instead, mages are better off as quick, ranged attack units, with staffs. The skills I use for the Mage are: Magic Attack, Freeze, Fireball, Electrical Storm, Flickering Flames, and Invisibility. The mage's other abilities are either close range (perhaps viable in the late game but an easy way to get yourself slaughtered in the beginning) or not particularly useful (especially Cosmic Prison). I'm not a particular fan of either of the passive abilities, but usually I side with Spell Mastery. When in combat, mages should be in the back. I like to use the Ranger-Mage combo in combat, where my ranger and my mage kill a unit before they even begin to attack. A Pierce and a Fireball will work wonders against enemies. Mages are also very good with poison damage.
Clerics: The cleric is an INDISPENSABLE part of any team, not just this one. His healing abilities will save your ass plenty of times. Clerics are one of the weaker classes, a lot of the time because their stat points are being diverted not only to strength but also intellect. Still, they are useful to mop up weakened enemies when your team is all healthy. The skills I recommend are Melee Attack, Heal, Smite, Heal All, Revive, and Fervor. I very strongly recommend spending your surplus points on Benediction; it will save you many a time. As for his other skills, purify is probably the most useful, although a properly powered Benediction renders it superfluous. Your first priority as a Cleric is to heal your teammates, then to attack. Letting your Cleric die is disastrous, but should it happen, that's why I recommend having a healing Ranger with you.
Warrior: Warriors are the best unit for just dishing out pure, unadulterated damage. Warriors can use a very wide variety of weapons. I strongly prefer Axes, but if you get a Hydra's Tooth in The Book of Dread and complement it with a good shield, it works just as well. Warriors, however, are quite slow, so most often you will use them to administer the killing blow to weakened enemies. Skillwise, Warriors are a pretty adequate bunch. I use Melee Attack, Power Attack, Bloodlust, Resolve, Cleave, and Inspire. Execute is an awesome attack against bosses, but generally it's not worth wasting the power on for mere peons. Warriors have some of the best passive skills, Leadership and Armor Proficiency. Either one works; I use Leadership to give me that final finishing move if I'm short. Warriors should definitely be in your front ranks, cause they can take a lot of damage. Damage reflection is especially useful in a warrior; doing more than 100% damage back to your opponent when they attack you is an awesome advantage that shouldn't be avoided.