Level-based systems have an inherent advantage over others in that encounter difficulty can be approximated within a few degrees. Unfortunately this also becomes a weakness in certain circumstances, like preparation. I've made a great many adventures for games which never saw the light of day. Later, those challenges proved to be too high or too low for groups I was running. With this experience I've found that the way with which I prepare games could do with some working over. The encounter difficulty is variable on at least three different factors: quantity of creatures, quality of said creatures and party advantages or disadvantages.

It's not hard to see that twelve goblins instead of just two is a more difficult encounter for most parties. Sometimes all that needs to happen to even the odds for a challenge is to play with the quantity of creatures being thrown at your adventurers. Note that this isn't always the case, though. Prime examples include when the party easily overpowers each individual foe and vice versa. Putting another peon into the foray who poses a miniscule threat to the players won't do much to scale the encounter. More often than not, simply changing up the mix of creatures - maybe more goblin lords than goblin grunts, for instance - can calibrate your challenge to suit whichever party meets it.

On the difference between goblin lords or goblin grunts, simply leveling your foes to more closely match the party could suffice. The quality of creatures that the adventurers challenge makes all the difference, most often more, than the quantity. A powerful spellcaster is much more dangerous, usually, than his much less powerful lackies, no matter how dense their numbers are. Preparing a bestiary is most useful in this endeavor because then you can fish from there for enemies instead of needing to flesh them out. In my games I've found that three different tiers of the same foe suits long adventures very well. If you haven't the time or patience to compile your own bestiary then there are plenty of resources for many systems on the internet.

Another telling factor in encounter difficulty is the advantages and/or disadvantages of the party. A fight with twelve well-matched fighters becomes more difficult when the party is surrounded by them but much easier when the party is beyond their reach and attacking them with ranged weapons. Warriors who fight with reach weapons aren't as hindered by unfavorable terrain as a battlefield because they don't need to cross as much distance as those without. Spellcasters who are low on spells or know only all the wrong spells for the encounter are reduced to only their backup plans, which usually entails either escape or their items.

Worth mentioning is that if both the party and the enemy are faced by a disadvantage then it doesn't scale the encounter at all. The same goes for an advantage. In order to give the adventurers a leg up you need to make sure that they're better off than their foes. Ruining archery by adding high winds and low visibility will only help the party if the foe's main strategy included bows. However, if the enemy they encounter is much more powerful than they are but already wounded, a very fortuitous battle can ensue for your adventurer's. The best way to tell if something will be a boon or a bane is to playtest it beforehand with as many details as you can manage to replicate. Happy encounter-preparing!