How to play Mages
This guide covers the use of any unit with high cost, high magical ranged damage, low melee, and low hitpoints. In particular, it covers the human Mage, and the Dark Adept.
These units can be very effective in the right hands, but they can't do all the work of an army. Their high cost and low hitpoints force them to be used only as utility units, not as the front line or core of an army.
When to use Mages
Anticipate who you are going to fight. Mages are often useful to exploit a particular weakness of the enemy (e.g. fire vs. Undead), and if you can arrange a battle with an enemy weak to your mages, get a mage for it. (In campaigns, you may have a large enough army to warrant multiple mages.) At least in multiplayer, it is important not to recruit a mage except for a specific purpose.
The Mage is the utility of your army, the unit that picks off those annoying wounded units in villages, or softens up the enemies that would otherwise have too much HP to kill. It's not a unit that goes around destroying enemies on its own, so make sure it has an army to be a utility for! Never recruit a mage when your army is in a tenuous position, because it may not be there when your Mage arrives.
The Mage should be the first unit you recruit. It is not a reinforcement. It is the hub around which the army turns.
Depending on how much gold you have, and how costly your mage is, you may be able to recruit 3 or even 4 mages. This group can be very powerful, but only if your opponent doesn't have units that can stop it. For instance, don't recruit three Dark Adepts if your opponent has Cavalrymen. The key to their usage is that they be able to break through any enemy; even if the enemy has a ZoC-line of units that can't be attacked from more than two hexes, the mage-gang is next to useless.
Of course, it's still as good as three archers, but remember: The mage costs more than the archer. If you want to win with it, you have to make it perform better than the archer.
Mages as attrition warriors
In addition to being magical, your mage can usually do more actual damage than any other unit in your army. Thus, if you run into a situation - for example, a tight passage - in which the available hexes, rather than the available units, are the obstacle to progress, then it's obviously useful to bring out your greatest damage-per-hex-occupied unit. The important thing in this situation is to never let the mage be attacked from more than two positions, and preferably not more than one. If the mage actually dies in the battle, all its value is wasted.
Specific weaknesses / usage in Default
The Undead are all weak to Fire. It's almost always a good idea to use a Mage on them; they're effective against Skeletons, Ghouls, Walking Corpses, and of course, Ghosts. If you see a ghost, you need a Mage to deal with it. (Exception: If the enemy gets lots of Dark Adepts and/or Skeleton Archers, don't. Mages are weak to those units; get Cavalrymen or fighters if Dark Adepts are a problem.)
The Drakes are all weak to Cold. The Saurians aren't so weak, but they have such low HP that Dark Adepts can kill them anyway. Don't get greedy and go for all Dark Adepts though, because enemy Burners and/or Skirmishers will cream you. You need something else to deal with them, like Skeleton Archers (cheaper, more HP, resist Skirmishers a lot, and Drakes are weak to pierce anyway.)
If a Rebel player uses Woses, you may be able to dispose of them with a Mage. If a Loyalist gets Heavy Infantry and Mages to face undead, massed Dark Adepts can massacre them. On the other hand, if your opponent has Horsemen or Ulfserkers, be very wary of using mages yourself.
How to use Mages
Unlike many units, Mages have noticably different usage in campaigns as opposed to multiplayer. In campaigns, because of their high experience requirement and low HP, it's important to keep them safe and feed them lots of XP. In multiplayer, because of their high experience requirement and low HP, it's important to keep them safe and to not waste XP on them. (well, actually it isn't that important, because you're not that likely to level up your other units anyway.)
General campaign usage
The mage is not your cannon fodder. It also is not your damage-dealing grunt. The mage is your spoiled little brat who gets all the experience that he didn't work for. He's the upper-class combatant who looks down his nose at the common grunt as he steals its kill for the common good.
Soften the target with a grunt, kill it with a mage, and fill in with grunts so the mage doesn't die before the next turn. Ignore the mage's ability because its only role is to kill wounded units, and you often don't have a lot of choice which units are open for killing.
Of course, you can't do this if you're low on units. If you're low on units, you probably shouldn't have used the mage in the first place. Even high-level mages have no business trying to fight the main battle without a substantial escort.
(Exception: If you're fighting Undead, or you have a lot of gold, you can summon hordes of mages and use them as grunts. Even if you do this, you should feed kills to and protect any mages that start getting XP.)
Advanced mages in campaigns
The Mage reaches true usefulness at level 2, when you get the choice between White Mage and Red Mage. The White Mage is a fine choice, but I won't go into detail, because this guide is not about healers. (Summary: You want somewhere between 3 and 8 advanced healers by the end of the campaign. Choose White Mages if you are behind on healers.)
The Red Mage is a general purpose unit. Use it to deal out 8-4 damage to any and all enemies. Use it to soften up high-defense targets, and give it kills occasionally when it's easy; even if you don't pamper it, you'll be amazed at how quickly your Red Mage will level up. Then, you'll be faced with another choice: Arch Mage or Silver Mage.
The Silver Mage is the specialty unit. Get it only if you're going for a teleporting assassin squad or if you care more about softening up the enemy than killing him. (The Silver Mage is also faster and resists fire and cold; there are some uses.)
The Arch Mage, you shoudn't even try to give kills to, ever. In the course of raining down awesome fireballs on the enemies, he's bound to pick up experience plenty fast; you can't keep him from killing if you want to! Eventually, almost no matter what, he will become a Great Mage, with a 16-4 attack and (relatively) tons of HP.
Once he becomes a Great Mage, don't recall him unless you really have to. Great Magi cost tons of upkeep and waste kill XP all the time. Pull him out in very hard scenarios, when you need his firepower, and in the final scenario, when it's time to pwn things.
The Dark Adept, unlike the Mage, doesn't reach a truly powerful state until level 3. At level 2, he gains enough HP to be left in the open sometimes, and enough attack that he's better than a level 1 unit for killing units, but he isn't really a power on the battlefield. Keep feeding him XP.
Then, you get a choice. It's not very significant. Both options are still dedicated mages with no poweful new abilities. The only real difference is that the Lich has a powerful new weakness. Get the Necromancer if your campaign likes to throw fire and holy units at you; otherwise, get the Lich. It drains and resists several damage types, making it potentially hold up to more damage, and has a slightly stronger ranged attack.
Oddly enough, what the Dark Adept eventually becomes is good at holding the line and softening up the enemy, rather than dealing heavy damage. It doesn't even do twice as much damage as the original Adept.
Use it for this. The Necromancer is the only Undead option for holding the line against fire and holy units, and it's not half bad against other units either.
Defend your mage. Don't let it die. If your mage dies, not only have you lost 16 or more gold worth of units, but also, you're screwed.
You can resupply your army with other units. Other units are just everyday units; you can interchange them. You can't resupply your army with mages, because the mage dies in one turn, and if your army spends even one turn without a mage, it could spell its demise.
It's not actually that earth-shattering, but defending the mage is a priority. The easiest way to screw yourself, with a mage, is to put it where the enemies can chop it up.
To wit: NEVER send a mage out by itself. By itself, it's no better than any other unit, so what did you spend that 4 extra gold for?
In a battle, though, it is of utmost importance. The mage is what lets you attack the one enemy unit on a mountain, rather than working around it. The mage is what lets you have reasonable surety at finishing off that 4-HP unit on the village. The mage is what lets you massacre that Drake Clasher when Night comes around (or that Skeleton when Day comes around.)
The two keys to using mages are to find a weakness, like high defense without the hitpoints or ZoC to hold it up, and to find a place where you can exploit that weakness while still being safe next turn.
Decide where your Mage is going to attack from, and re-form your whole army as a protection for it. Put the Mage where it can be defended while attacking, and put its guards where they can defend it while attacking.
Mage-gangs in multiplayer
It's impractical to defend three or more mages at once. No matter what you do, the enemy can use his whole army to break through somewhere and kill a mage.
The important thing is that a solid wall is not the only defense. The important thing is to make sure that if the enemy does this, he has to at the same time lay his whole army open to the attacks of your remaining units. There may be a route in, but let there be no such route that does not leave the enemy in poor defensive terrain and formation.
Being unable to kill a mage without losing the game is just as effective as being unable to kill it at all.
How to play Loyalists article
How to play... series