Especially when dealing with a party of mixed levels as can occur with a West March style adventure, encounters are daunting for a GM. The Challenge Rating system assumes a balanced party of 4 players at the same level. Often, it seemingly does not fully account for special abilities, or specializations that the players may have taken. This can result in some strong encounters being overly simple, and some weaker encounters resulting in party death.
The first thing when designing encounters is to assess the objective. Is the aim to drain resources? Provide insight into what is occurring in their environment? Or a major plot point? Not to say an encounter can’t be all of the above, but for the purposes of this, we shall assume only one is desired at a time.
A good example of encounter process would be Left 4 Dead (1 or 2) when played on a high level of difficulty. The common infected slowly chip away at health, consuming ammo, temporary health. Sometimes stashes exist to resupply. Sporadically through the map, the special infected will attack, and cause much more ammo and health to be invested. Finally, a tank encounter may take almost all the bullets.
Resource chipping encounters are looking to get the players to spend one or three spells, maybe a X/day power, and take some hit point damage. This creates the need for the group to assess if they have the strength to keep forging forward. However, this is not to say the same group over and over again is the way to approach it.
In your average group, each player is likely to have things they excel at. Some may do incredible burst damage, while others do consistent but reliable damage through spells. To keep the players engaged, these methods need to be checked, and catered to. Yes, Catered To. If a character excels at something, but never gets to demonstrate that focus, the player becomes uninvested in the character and they will cast Sword (or equivalent) until all fights are resolved.
So, the wizard that specializes in using spells in combination to cause greater effects like grease and burning hands? An encounter should be designed where it is unlikely that the party’s martial characters can resolve, but have such abysmal reflex saves that the grease and fire will certainly resolve the issue.
Environmental difficulties can spruce up a low encounter fight. Rooms are not independant blocks of space and time. When allies are fighting nearby, the sounds are likely to attract others. Once verified that there are issues, why wouldn’t the new combatants try and circle around to press the party? Casters hide in the back for a reason. When the group suddenly has to split their focus, the encounter’s Challenge just increased. The players may retreat into a corner trying to draw both groups into a cone in front of them rather than the two fronted combat they have.
Sometimes its worth pointing out a flaw or issue in the group’s capabilities. If the group has a thing for will and fortitude saves, some undead will cause real problems for them. An overly strong character that faces a creature which slowly takes away their strength from a distance, while something else engages and blocks the strong tank can create quite a challenge for a group.
When using faceless enemies to fill an encounter out, the type and some of what is found with them can be just as plot relevant as the trusted LT of the evil overlord. If the relatively unknown manipulator behind a series of events that the players are following has gotten gnome rogues to attack the players this has a different meaning to some human street toughs even if as a GM you might use the same stats (minus weapon damage). The players might think to go investigate the gnome connection, and there is a political ploy that is present. With the human street toughs, one of them seemed to be stronger than the others (a leader of some description) who has a well faceted gemstone in his pouch, presumably as payment. When touched to blood, it shatters causing damage after a moment. The players now suspect advanced magic, and don’t trust gems.
A third option would be that two of the thugs look oddly similar. Asking around, leads to information about the twins. As known street toughs, their movements were more easily followed. Fourth option is for those who wish to upset their players. Players often take a ‘Keep what you kill’ attitude. Throw some items into the pouchs that could be heirlooms. Then have in a couple sessions the family of the deceased seeking vengeance.