I checked every sourcebook available from the recent Warhammer fantasy bundle (check this link for a list of books), there are no official rules for magic duels. These duels are only briefly mentioned as flavor text.
However, magic duels are mentioned several times on Realms of Sorcery, the splatbook about spellcasters in general. Here (page 78), they describe the process necessary to upgrading your wizard rank, where the apprentice wizard will have to battle a senior Magister before he is accepted as a Magister himself:
Becoming a Magister
At the end of his travels decided at the master’s discretion, a Journeyman Wizard returns to his studies with his master. During this period, the master discerns the level of arcane mastery and control his apprentice has, as well as whether he has become tainted in any way by Dark Magic. This process is long, but once the master is satisfied with his protégé’s skill and purity, the Journeyman Wizard will be expected to travel (sometimes with the master, but more often without) to the master’s home College (if they are not already situated there), to study the deeper secrets and more dangerous magic of the
Order. The College is also the only place where an apprentice may be accepted as a full Magister and given all the rights, privileges, and duties of the Order.
If his request is accepted, the Journeyman will have his arcane skills pushed and tested like never before. The clarity of his arcane senses will be tested, as will his control, speed, and ability to channel the Order’s distinct type of magic. Finally, his spellcraft will be tested through a series of trials, ranging from relatively easy tasks to a magical duel between himself and an examining Magister. This duel is not fatal, and the apprentice will not have to win, though he will be highly esteemed if he does.
This duel is apparently not lethal, as the goal is simply to find out how much skill the apprentice has developed. As such, you can be certainly safe to assume that attack spells are either not used, or that the examining Magister has the proper safeguards in the form of protective spells and/or talismans.
For completeness, there is a mental duel spell on the fantasy wargame, but not even remotely similar to what we want here. And just for idea gathering, I also checked several 40k rpg books, as they are similar enough to fantasy 2e that some rules could be grabbed, but there are also no kind of psychic duel other than a wargame power from chaos marines.
House Ruling a Magic Duel
If you want magic duels to be as exciting as physical combat, simply use the same rules for physical combat, but that only applies when wizards are dueling. Instead of weapons, they wield spells and raw magical energy created from their mastery of the winds of magic. These rules (for physical combat) have been tested and are known to work, and at most, all you have to do is re-flavor the names of some actions and they are ready to go.
Here is how I would do it. Each relevant physical secondary stat would be converted into a similar mental secondary stat, and actual damaging spells are not allowed, but other spells would work as normal.
- Initiative: 1d10 + Intelligence (instead of Agility)
- Making an attack: 1d100 using Willpower (instead of Weapon Skill)
- Avoiding an attack: 1d100 using Intelligence (instead of Dodge or Parry).
- Causing damage: Each successful attack causes an amount of damage equal to your 1d10 + Magic ability.
- Absorb Damage: Each attack's damage is reduced by an amount equal to your WP/10 (equivalent to Toughness Bonus). Ex: 51 WP reduces 5 points of damage on a Magic Duel.
- Aim Spell: +10% on your next attack.
- Attack: Conjure an attack (see above).
- Cast: Conjure a non-damaging spell (works as normal spellcasting).
- Counterspell: Ready a defensive check using Intelligence to avoid an attack with +10%. (This is similar to a Parry or Guarded Attack)
- Magic Trick: Opposed Intelligence Test, if you win, your next attack cannot be avoided and he is prevented from using the All Out Attack or Defensive Stance actions until your next turn. (This is similar to a Feint)
- All Out Attack: You make an attack with +20%, but cannot avoid attacks until your next turn.
- Defensive Stance: You do not make an attack, but gain +20% to avoid attacks until your next turn.
Remaining actions are unchanged. But drawing a sword or crossbow during a magic duel is considered dishonourable and the duel is immediatelly interrupted.
You will notice that this is similar to them using the weakest damaging spell at each other, Magic Dart, while conjuring other spells to hinder each other or deflect each other missiles. Also, both attacking and defending yourself would use the Channeling skill, which represents how well you manipulate the energies from the winds of magic.
For the sake of avoiding the Tzeentch's Curse (which should be in the College's best interest), the arena is on a large permanent Protective Circle that prevents most harmful chaotic energies. So each spell cast would benefit from a Protective Circle (page 144 of the core book), as such, they would -re-roll one d10 when conjuring spells on a duel and a double, triple or quadruple is rolled.
At first, I also thought that Intelligence was underused, but the game has fewer mental stats than physical stats, so creating an analogous stat for each combat stat proved difficult. Intelligence, since it represents a character's cunning as well, does work best for coming up with defensive tactics on the fly. While willpower is used to force your will against the other wizard. When not directly attacking each other, they are free to conjure other spells to attempt to gain advantage over each other.