Nox

Play it through three times: once for each class. They're unique enough that it's fun to play through the game again.

Conjurer

The best conjurer spells are Burn (for zombies), Charm (if you have the right Beast scroll), Fist of Vengeance, Force of Nature, Greater & Lesser Heal, Meteor, Pixie Swarm, and Stun. Pixie Swarm will probably be your go-to attack spell for most of the game, as the pixies hone in on enemies like the Wizard’s magic missiles.

Unlike the Wizard, you WILL get a lot of use out of your "trap" ability, the boomer. Boomers (or whatever they're called in the game) will run around in search of enemies. When they find one, they detonate and (hopefully) release the three spells you put inside them on their prey. You can use them to take out large groups of enemies by giving them area-effect spells, or difficult bosses by loading them with Fists of Vengeance or Force of Natures. Remember, including Stun, Poison or Slow as the first spell pretty much guarantees that you'll kill the enemy.

There is no "best" combination of beasts to summon. Some, like Mimics and Demons, are almost always useful, while others, like Will'o'Wisps and Ghosts, take some practice. The best rule is to generally summon beasts which complement your fighting style. If your conjurer uses a melee weapon and rushes into combat, summon beasts with ranged attacks. If your conjurer stands back and fights enemies with a bow or crossbow, summon some tanks (like Ogres) to keep the enemies at a distance.

The warrior is the hardest class to play well (unlike most of these kinds of games). You have a very limited selection of abilities (5, as opposed to 25 and 45 for conjurers and wizards respectively), and two of them are pretty worthless. With the magic classes, you have way more options to fall back on, whereas warriors can pretty much only attack or retreat.

When fighting wizards and conjurers you'll definitely feel a disadvantage. To overcome this, you'll almost always want to be fighting up close. If your enemy is surrounded by other hostiles, use your harpoon to reel him to you. If your enemy is alone, use your charge to take him out. Avoid getting caught in the middle of a large melee you cannot wi - ring the various fighters to you to take them out more easily.

The best spells are Burn (for zombies), Death Ray, Dispel Undead, Energy Bolt (for continuous damage), Fireball (for its blast radius), Force Field, Fumble, Mark & Teleport, and Magic Missiles. The other spells are all useful insofar as there is at least one valid use of every spell in the game, but the ones listed above will probably be the ones you rely on most often.

Wizards can only use staffs for weapons, and more often than not you'll want to use a spell instead of using the charges in your staff. Basically, ignore the Sulphurous Staves and focus on finding the Fireball Staves. They'll be rare at first, but then you'll find enough of them that you can use them pretty liberally.

Sadly, your ability to make traps won't be very useful. If you're being chased by enemies (like undead or something), it's handy to lay down some traps with Dispel Undead in them but outside of these special situations you won't have many opportunities to use them. The game is always pushing you forward, whereas these traps require extensive backtracking and sneaking to pay off.

The Resistance (poison, fire and electricity) gear is useful in certain situations, but is usually outdone by the Speed or Healing gear. I mean, if you're in the Poison Swamps or the Fire Caves then dress accordingly, but otherwise stick with speed/heal.

Resistance gear comes in almost all armor types (i.e., helmet, boots, leggings, etc.), while Speed gear only comes in boots, and Healing gear only comes in cloaks and tunics/cuirasses.

The crossbow is probably one of the best weapons in the game. It does an insane amount of damage (even before fire/poison/shock bonuses are added from the weapon itself and the arrows it uses) and both conjurers and warriors can use it. The real drawback is the slow reload, but a conjurer should have some tanks to keep enemies at bay. Warriors may want to opt for a melee weapon for this reason.

The Staff of Oblivion, in its various stages, is possibly the best melee weapon in the game for wizards and conjurers. Warriors will want either the great sword, or a one-handed weapon and a shield for maximum defense.