One more commentary on this:
Assuming a fairly double tight line formation, and a clvl 5 fireball with a save DC of 15, and warriors with 10 hp each (12 con and Toughness), you will only kill about 6-12 soldiers per fireball and 2-5 will be severely wounded. In general, you will probably be able to save at least half of the wounded soldiers if your men are trained to act quickly. So, each fireball on the other side is going to take out a fair portion of your 100 man army of 1st level warriors.... but if your army is 500 men, you can probably take a 5th level evoker's entire arsenel of fireballs and still have taken only moderate casualties. (All things considered). If you're fielding 2nd level warriors with 16 hp or so, it won't be unusual to have no deaths from a fireball.
The next question is this: why are you fielding 100 first level warriors and what other resources do you have. For that matter, what resources does your enemy have?
I would imagine that your enemy can probably hire 100 first level warriors with bows or crossbows for the same price as his 5th level wizard, so maybe you're not too worried. After all, the wizard will run out of spells before the archers run out of crossbow bolts. And it's easier to take out one wizard with your archers/wizard than it is to take out 100 archers. If economics are the determining factors in force composition, you may well find it easier to defeat the fireball tosser than ordinary troops that would cost a similar amount of money. Fireball rules low-level, small unit skirmishes (of course, so would a 5th level fighter/barbarian with whirlwind attack or great cleave). As the troop numbers increase, however, it becomes less and less significant unless you field units of mages who each toss a fireball. The spells that will change a battle are ones like sunburst, firestorm, and summon monster VII (huge elementals--especially air elementals in whirlwind form).
Other tactics: For every hundred men in your army, assign ten as spell disruptors: readying actions to fire their bows/crossbows at the enemy spellcaster. (This can be defeated by protection from arrows which can, in turn be defeated by greater magic weapon).