Enemy unit types: Heavy Melee, Light Melee and Light Range
Enemy units: Orc General, Ancient Orc, Bandit, and Dryad
Your key unit: Golem (Elve), Mortar (Human), Orc Strategist (Training Ground)
Early game strategy
Key unit is will depend on enemy troop mix. Your Archer and sword dancer are strong against 1 enemy unit, neutral to 1 and weak to 1. A 2/3 mix is your likely approach favouring your unit that is best against the greatest number of enemies. For fights with a lot of Orc Generals if you can clear the enemy support and are left with your archers verse the Orc General you have a clear advantage. In Era 2 your Heavy Melee (Paladin/Treant) units will be good for fights with lots of Ancient Orcs and Bandits.
Mid to Late game strategy
Heavy Range (Golem/Mortar/Orc Startegist) are your key units. They have no or minimal defense bonus against light melee so focus on taking out the ancient orcs first.
Support options
Light Range (Archer/Crossbowman/Ranger/Dryad) or Mage Units (Sorceress/Priest/Blossom Mage/Banshee) provide support against the Heavy Melee units but are likely to be targeted first due to highest initiative. You will need the key units to help protect them from the Ancient Orcs, Dryads and Bandits. For encounters strong on heavy melee units these are your go to support choices, especially the Dryad and Blossom Mage
If the fight is strong on Light Range units with minimal Heavy Melee consider Light Melee units (Sword Dancer/Axe Barbarian/Cerberus/Drone Rider) as support units.