Whilst creating earthquakes or summoning lighting may be too energy consuming, we can instead use nature to our advantage.
Rather than creating an earthquake, it is far easier to engineer a landslide. Your witch would stand at the top of a canyon, mountain passage, hillside, cliff, etc. When the enemy army approached, she would use her magic to push dirt or rocks or even snow onto them or in front of them to block their path. You could even have two witches, one to block their way forward and another to block their retreat, trapping them in the middle. From there archers can fire down onto the army trapped army to kill them.
In a similar sense, you could have witches create floods by bursting dams or riverbanks, causing the water from a reservoir to flood the valley or the river downstream, thus drowning and battering the enemy.
Air can easily be utilised for an ambush. By pushing loose material, such as dirt or sand, into the enemy army you have effectively blinded them. The small particles will go into the solider’s eyes, causing them to water and be extremely uncomfortable. This allows your own soldiers to run in and kill them as they won’t be able to properly defend themselves.
Fire is by far the easiest element to use as it spreads on its own. This means you do not need a huge amount of it to achieve a devastating effect. When the army marches through a dried grassland or forrest, your witch can create small fires around them. The dried land will quickly ignite and spread, eventually engulfing the army in flames.
All of the above depend on the local environment, you cant create a flood without a large body of water, nor can you create a landslide without somewhere to push the material down. These methods can be more easily employed in a variety of places.
While you may be hard pressed to find a mountain to create a landslide on in a city or village, this does not mean you can’t employ other methods. Rather than pushing rocks down hills, you can instead collapse walls and buildings by damaging the load-bearing walls. This achieves the same effect as described above, just in settlement.
Water is hard to use if there is no natural body. However, it is not impossible. You can bring water and heat it to create steam, using it to obscure your army’s movements. Quite a valuable tool in some cases.
Air is also hard to utilise, though you could potentially use it to create updrafts, allowing you to use gliders to scout out the enemy or drop troops behind their lines to sabotage equipment.
Finally, we come to fire. Fire is again the easiest to use, enemy siege equipment or tents can be set alight, killing or weakening their forces. You could instead use it to create smoke, obscuring your army’s movements.
Here are some ways your witches can utilise their magic in an ambush-like way. Some of the above may instead fall into sabotage or skirmishing but i feel like that is what you were going for. None of these methods would require a huge amount of energy as most are using the already existing energy found in nature, such as gravitational potential or chemical energy. This allows for witches who’s magic is not exceptionally powerful to be very useful in some situations.