Answer to this will really depend on a few things:
- How effective is the fireball and can armor offer protection? Would a wall of shields prevent a fireball from getting at the soldier on the other side? Is being hit by a fireball 100% fatal?
- Effective range of a fireball. Which has the longer range, arrows or fireballs?
- How numerous are these wizards (1 per army? 100 per army?) and how frequently can they throw fireballs (daily limit vs constant attack?)
The answer to these questions will ultimately determine tactics...wizards of this extent tend to favor the undisciplined forces more than well disciplined ones.
Undisciplined is a reference to skirmishers or barbarian type tactics where the unit fights individually or in a spread out area. Barbarian type warriors don't work well with others...in tight formations their attacks tend to interfere with each other as much as anything, so they tend to fight as scattered groups and put emphasis on individual heroism. Skirmishers also fight in loose formations, firing or throwing their weapons and then falling back. This tactic would be exceedingly useful as a method of disabling enemy wizards (range of wizards comes in here, if archers can get close, fire their weapon in the direction of wizards, then fall back before melee troops can engage them...you have an effective method of disabling wizards). A fireball would likely be deadly to the few caught in the fireball, but the loose formation prevents a total loss scenario that tight formations would experience (unless you wizard can machine gun fireballs with little regard to the number cast in a day)
Defence in depth also becomes more valuable, spreading defences deep instead of one layer. 3 walls instead of 1 giant wall. Destruction of the first wall is inevitable, but falling back to the second wall and defending as the enemy comes through the first wall is a great tactic. One thing of note...fireball range is usually less than a longbow or similar archer. Depending on your wizards range, the ability to send fireballs flying at a castle wall may endanger the poorly armoured wizard and put him in the range of archers. And another note...opposing wizards on castle walls can just as easily send fireballs back at attacking wizards. Defending wizards here have a great advantage...they can spot the attack wizards and attempt to disable them far easier than the attackers can spot them, they have a castle wall to hide behind, and if it's a layered defence, they can always fall back and defend from the second wall (or third, or keep, or fortress, etc...) as well. Remember defending wizards can be more dangerous than attacking ones.
Protracted sieges would be far less frequent. One of the larger time wastes in any medieval campaign is putting together the tools required (catapults, seige towers, rams, etc...) and the slow whittling of a castles defences. Since wizards could work as siege weapons to disable walls, the tactic of hiding behind walls with a far lesser force and forcing a protracted siege while reinforcements arrive is no longer a viable solution. Actually has the effect of making a rampaging army far more effective as the need for long sieges is gone
Disciplined troops is a little different as the size and strength of the fireball comes into play. Instead of relying on spread distances to reduce the effectiveness of an area effect attack, they are going to try to use each other to resist the attack. Heavy mail and armour, with thick padding underneath to reduce the effect of a fireball is most likely, along with a large shield. These units rely on each other to protect themselves, not just themselves...the front rows holding shields forwards with back rows holding them above. You don't give much information on how strong these fireballs are...so it's hard to tell if this is an effective attack.
Cavalry generally fall into two categories...fast and light cavalry will use their speed and agility to try to avoid and dodge incoming attacks, while heavy cavalry rely on their status as 'tanks' on the battlefield. Need more details on fireballs here to determine their effectiveness...though I would assume fireballs would have a pretty devastating effect on heavy cavalry. That said the best counter to skirmishers is generally cavalry as they can easily ride through loose formations and cause havoc.
Horse archers might be exceedingly effective...riding up, firing an arrow towards enemy wizards, then retreating while reloading to repeat. Cantabrian circles would ensure a stream of arrows flying at a wizard (or group of wizards) while maintain a pretty wide spread making area of effect attacks far less useful.
Gives an interesting loop... Skirmishers counter wizard, cavalry counter skirmishers, tight formations (spears) and heavy cavalry counter cavalry, wizards counter tight formations and heavy cavalry.
Missed the poison gas section...
Poison gas is a hard tactic to use effectively and weather variations can make it as brutal on yourself as it is on them. It's easier to use in a defensive sense as the enemy has to advance towards you to attack and through the poison gasses to get there. Attacking with gas has the bad tendency to take down your own troops as they advance. Not sure what effects this could have on battles, tight formations on attack are obviously at the disadvantage here.