One of the most important things to consider is the you need to balance the strength of the party against the enemies.
An acronym I typically use is cwpa. (Commensurate With Party Ability)
What happens when two equal forces collide? The one willing to expend 100% of resources will barely come out the victor. Random chance plays too big a factor into success and lessens the impact of the decisions and tactics they choose.
If the party encounters a force lower than 1/4 of their total power it can used as a near certain victory to boost morale and will use none of their resources. If they encounter a group 1/2 their power it will use approximately 1/4 of their resources. This how you can craft the tension and raise the stakes to lead to a truly momentous battle where the decisions of the player s are the core components to their victory or failure. This will make to the most of the game and lead to a memorable session for the players and the GM.