Well this setup is very broad, so it would vary a lot depending on the geography, troop composition, supplies, and various other factors. I will attempt to discuss a handful of potential options depending on the circumstances though.

In terms of taking advantage of geography some tactics that would be advantageous would be to position your army on the high ground, position them with the sun to their backs, or position them with the wind to their backs. John Hawkwood is often praised for having implemented all 3 of these tactics simultaneously to engage a much larger and better trained/armed army at the Battle of Cascina, even though he still ultimately lost.

Depending on how well supplied you are, you may be able to fortify your army behind ditches, spikes, and earth mounds, then wait until your enemy is forced to engage you on your terms. This is especially effective against cavalry based armies since it undermines their advantage of mobility, but relies on you having enough supplies to outlast your opponent, otherwise they could simply starve you out in a siege. Flemish militia effectively used this tactic to defeat French knights at the Battle of the Golden Spurs.

If you are on home territory then you may be able to employ scorched-earth tactics in which you draw all resources and troops from the surrounding area into fortifications, then regularly harass the enemy army with skirmishes until they run low on supplies and are forced to withdraw. The Hungarians soundly defeated the Second Mongol Invasion of Hungary using this tactic.

If the enemy army needs to travel through a narrow pass or bridge that can't be easily circumnavigated then you may be able to station troops at it, which would then negate their numerical advantage since only a small number of troops would be able to engage each other at any given time. However, this relies on your troops being of at least equal or greater quality to those fielded by your opponent. William Wallace used this tactic to successfully defeat a larger English army at the Battle of Stirling Bridge.

Depending on how much cover is available in the form of things like forests or hills you may be able to use guerrilla tactics to stage a trap or ambush. One way this could be accomplished could be to hide a portion of your troops behind some form of cover on either side of a clearing, then luring the enemy army into it with your remaining troops. Alternatively, you could attempt to shadow the opposing army and wait to attack until they find themselves in a disadvantageous position such as traveling through a ravine, crossing a river, or encamped for the night. These tactics work best when your enemy is unfamiliar with the geography of the land. Skanderbeg used this tactical style to regularly defeat invasions from significantly larger Ottoman armies into Albania for 25 years.

There are a number of tactical troop formations that were used during the medieval period which demonstrated themselves to be able to defeat significantly larger armies using other tactical formations, such as wagenburg or pike formations, but most of these require a specific troop composition and training style for your army prior to its deployment.