Many of the other answers focus on individual attacks, which when only dealing with a small number of soldiers is fine, but might be too slow to noticeably affect the battle. I think focusing on tactics that affect more than one soldier, or hinder the enemy commander or messengers will be more effective against an army.

##Death From Above Have your wizard pick up the heaviest rock they can and slowly bring it to hover in the air above a chosen target or group (enemy commander or messengers). If they don't notice it, you've just dropped a heavy weight from hundreds of feet in the air on top of their head.

If there is no single target, just have the wizards pick groups of enemies at random and drop the rocks, it's likely that it will hit someone.

##Fear tactics If they notice the rock, they'll have to keep moving to avoid it being dropped on their heads. This will at minimum prove a distraction for the enemy leader(s) and most likely also keep one or more scouts out of the battle (someone has to watch the rock and find out where it's going). With 10 wizards, the enemy commander (and strategists) might very well spend the entire battle avoiding being crushed by rocks, preventing them from meaningfully contributing to the battle.

This is still effective without a commander to target, since it will prevent groups of enemy soldiers from creating or maintaining a formation that isn't constantly moving.

##Bombs away This tactic is extra effective if your side has access to gunpowder or other chemical/alchemical explosives.

##Light in the eyes Another effective technique (if it's sunny) is to take a shiny piece of metal and use it to bounce sunlight directly into the eyes of the enemy soldiers. This will function as both a distraction and an easy way to blind the enemy soldiers at a critical point in the fight.

Edit: Additional ideas

##Flint and Steel See if your wizards can discreetly get flint and steel (or some other kind of fire starter) across the battle field. Then have them set to work attempting to light anything flammable such as wagons, barrels, torches, tents, or food supplies. They might not get many before people notice, but the effect will range from "distracting a few soldiers" to "distracting a lot of soldiers and burning a lot of enemy supplies".

Many of the other answers focus on individual attacks, which when only dealing with a small number of soldiers is fine, but might be too slow to noticeably affect the battle. I think focusing on tactics that affect more than one soldier, or hinder the enemy commander or messengers will be more effective against an army.

Death From Above

Have your wizard pick up the heaviest rock they can and slowly bring it to hover in the air above a chosen target or group (enemy commander or messengers). If they don't notice it, you've just dropped a heavy weight from hundreds of feet in the air on top of their head.

If there is no single target, just have the wizards pick groups of enemies at random and drop the rocks, it's likely that it will hit someone.

Fear tactics

If they notice the rock, they'll have to keep moving to avoid it being dropped on their heads. This will at minimum prove a distraction for the enemy leader(s) and most likely also keep one or more scouts out of the battle (someone has to watch the rock and find out where it's going). With 10 wizards, the enemy commander (and strategists) might very well spend the entire battle avoiding being crushed by rocks, preventing them from meaningfully contributing to the battle.

This is still effective without a commander to target, since it will prevent groups of enemy soldiers from creating or maintaining a formation that isn't constantly moving.

Bombs away

This tactic is extra effective if your side has access to gunpowder or other chemical/alchemical explosives.

Light in the eyes

Another effective technique (if it's sunny) is to take a shiny piece of metal and use it to bounce sunlight directly into the eyes of the enemy soldiers. This will function as both a distraction and an easy way to blind the enemy soldiers at a critical point in the fight.

Edit: Additional ideas

Flint and Steel

See if your wizards can discreetly get flint and steel (or some other kind of fire starter) across the battle field. Then have them set to work attempting to light anything flammable such as wagons, barrels, torches, tents, or food supplies. They might not get many before people notice, but the effect will range from "distracting a few soldiers" to "distracting a lot of soldiers and burning a lot of enemy supplies".

Many of the other answers focus on individual attacks, which when only dealing with a small number of soldiers is fine, but might be too slow to noticeably affect the battle. I think focusing on tactics that affect more than one soldier, or hinder the enemy commander or messengers will be more effective against an army.

##Death From Above Have your wizard pick up the heaviest rock they can and slowly bring it to hover in the air above a chosen target or group (enemy commander or messengers). If they don't notice it, you've just dropped a heavy weight from hundreds of feet in the air on top of their head.

If there is no single target, just have the wizards pick groups of enemies at random and drop the rocks, it's likely that it will hit someone.

##Fear tactics If they notice the rock, they'll have to keep moving to avoid it being dropped on their heads. This will at minimum prove a distraction for the enemy leader(s) and most likely also keep one or more scouts out of the battle (someone has to watch the rock and find out where it's going). With 10 wizards, the enemy commander (and strategists) might very well spend the entire battle avoiding being crushed by rocks, preventing them from meaningfully contributing to the battle.

This is still effective without a commander to target, since it will prevent groups of enemy soldiers from creating or maintaining a formation that isn't constantly moving.

##Bombs away This tactic is extra effective if your side has access to gunpowder or other chemical/alchemical explosives.

##Light in the eyes Another effective technique (if it's sunny) is to take a shiny piece of metal and use it to bounce sunlight directly into the eyes of the enemy soldiers. This will function as both a distraction and an easy way to blind the enemy soldiers at a critical point in the fight.

Many of the other answers focus on individual attacks, which when only dealing with a small number of soldiers is fine, but might be too slow to noticeably affect the battle. I think focusing on tactics that affect more than one soldier, or hinder the enemy commander or messengers will be more effective against an army.

##Death From Above Have your wizard pick up the heaviest rock they can and slowly bring it to hover in the air above a chosen target or group (enemy commander or messengers). If they don't notice it, you've just dropped a heavy weight from hundreds of feet in the air on top of their head.

If there is no single target, just have the wizards pick groups of enemies at random and drop the rocks, it's likely that it will hit someone.

##Fear tactics If they notice the rock, they'll have to keep moving to avoid it being dropped on their heads. This will at minimum prove a distraction for the enemy leader(s) and most likely also keep one or more scouts out of the battle (someone has to watch the rock and find out where it's going). With 10 wizards, the enemy commander (and strategists) might very well spend the entire battle avoiding being crushed by rocks, preventing them from meaningfully contributing to the battle.

This is still effective without a commander to target, since it will prevent groups of enemy soldiers from creating or maintaining a formation that isn't constantly moving.

##Bombs away This tactic is extra effective if your side has access to gunpowder or other chemical/alchemical explosives.

##Light in the eyes Another effective technique (if it's sunny) is to take a shiny piece of metal and use it to bounce sunlight directly into the eyes of the enemy soldiers. This will function as both a distraction and an easy way to blind the enemy soldiers at a critical point in the fight.

Edit: Additional ideas

##Flint and Steel See if your wizards can discreetly get flint and steel (or some other kind of fire starter) across the battle field. Then have them set to work attempting to light anything flammable such as wagons, barrels, torches, tents, or food supplies. They might not get many before people notice, but the effect will range from "distracting a few soldiers" to "distracting a lot of soldiers and burning a lot of enemy supplies".