Every encounter has to be challenging in some way. Otherwise, why bother? The trick is that not all challenges are created equal. I see many encounters that boil down to “it does a lot of damage” or “it’s really hard to hit.” You might be able to get away with this occasionally, but as a staple, your game will quickly start to go stale. The reason for this is that there’s a difference between mechanical difficulty and creative difficulty. In the former case, “hard” really means “unlikely.” It doesn’t task your players with anything more than just running the standard response program. Creative challenges, by contrast, force the players to think and adapt.
One example I use often is “Puncture,” an encounter power that triggers when the monster hits someone, causing them to take Ongoing 5 until the end of the encounter. The challenge isn’t that it’s more damage, but that it’s an unusual mechanic that makes the players stop and think about what they’re going to do about it. We’re four levels past that encounter and the power is still mentioned at least once a month. If the monster had just been about dice and numbers, they likely wouldn’t have even noticed, much less cared.
Tonight’s encounter is likely going to join the ranks of those that are memorialized in song and story. They fought giant millipedes with a nasty bite (ongoing 10 necrotic). The gotcha was that whenever they failed a saving throw, the ongoing damage could spread to a nearby ally. My players gave it a nickname and it rapidly became a joke to see who infected whom. The thing they’re most likely to take away from the encounter is trying to figure out the range and maneuvering to avoid infecting the most hurt characters.
Once you start looking at encounters in this way, there’s the danger of including too many of these mechanics, something I refer to as a “salad bowl.” The trick is to give them one or two things to handle that shape and define the encounter. Much more than that and things have a tendency to become chaotic and confusing. Make sure that these powers don’t have too much synergy (a power that immobilizes is generally not good with a power that punishes people for not moving, for example). It’s often helpful to decide from the beginning which monsters are going to be the focus of the encounter and which are there for support.
It’s also a good idea to “spin the table” before you finalize your choices, by which I mean that you should ask yourself what you would do if you were a player in this encounter, then ask yourself if you would enjoy playing out that response. It’s this last step that trips up many GMs, because what’s effective is not always fun. It’s also very important to make sure that the encounter centerpiece doesn’t prevent someone from being able to participate. This is most often an issue of terrain or line of sight, but other mechanics can be just as debilitating (large resistances/immunities are another common cause).
Kill them all,
The Evil GM