In my campaign, I find that I need to regularly use level + 1-3 encounters, or it becomes too easy. If the party is somewhat optimized, or if your players are good at tactics, they'll find standard encounters quite easy most of the time. When I play in other campaigns, we usually do encounters at fist level that give 150-280 experience each, so much higher then our level. This is a possibility to keep in mind, as well as the other suggestions about terrain and monster roles.
This is my experience too. Encounters below N+2 are a cake walk for pc's and just a way for them to get some cheap exp and or move towards a milestone. There's other factors, like is this encounter set in the wilderness where it's likely to be the only encounter of the day? An encounter in a "dungeon" is potentially a lot more dangerous because it can spill over into a second encounter and is much more likely to be just one encounter in a series of encounters (which compounds the effects if having used dailies). On the other hand, a wilderness encounter can place the pc's in a position of having little or no chance to use terrain to their advantage, since the pc's are frequently out numbered not having a way to shorten the front via some geographical choke point can put the pc's in quite a bit of jeopardy of being flanked and or surrounded.
The parties got two strikers and two leaders, and no controller.
So they should do well vs solo or encounters with only a few monsters.
Try minions and skirmishers in numbers. Higher level monsters to make it more dangerous/harder. Also consider terrain /tactics that separate the party up. Often when a party is spread out they are more vulnerably in 4E
Put some enemy controllers out there too. There's certainly a lot of ways to challenge a party. Replacing brutes with soldiers almost always creates a more difficult encounter. Throw 3-5 minion archers in an encounter without bunching them up and for the price of 1 std creature the monsters have added 5 attacks to their offense. Decrepit Skeletons are kind of wimpy and only do 3dmg with a short bow but 6 of them are the same exp value as normal skeleton.If you want to challenge a group of 5 level 2 pc's a group like this in the right geography (i.e. put the archers behind some sort of hard to cross terrain feature, or up on a ledge that can only be accessed by a flight of stairs.)
This will really test them.
1 blazing skeleton 200
3 skeletons 450
7 Decrepit skeletons 175
1 corruption corpse (zombie) 175
total exp: 1000, level 5 encounter (N+3 for 5 level 2's)
the blazing skeleton and corruption corpse both have solid ranged attacks with ongoing damage, and weaken. They also both have a significant aura. the 3 skeletons are basically to block pc movement and prevent the pc's from having their way with the decrepit skeletons. The zombie and blazing can both start as ranged attackers trying to weaken and set on fire as many pc's as possible while the skeletal archers pepper the pc's with volleys of arrows. a little focus fire from the 9 ranged attackers in this encounter could be deadly. (I wouldn't put all 9 attacks on one pc but 4-6 would be intimidating for most pcs). When the pc's get in melee with the skeletons the corruption corpse could move adjacent to one or more pc's using his aura to make the skeletons hard to hit.
The one thing you didn't specify is how many encounters in a day?
I finished a 24 themed mission with 4 fights that were Level +2, Level +2, Level +1 (with a tactical advantage to the monsters), level +4 (with tactical advantage to the heroes).
It was a close thing. Three guy were in the negative at the end, two had no surge left at all.
4 level N+2 encounters is a solid day of adventuring for most parties in 4e and I'm sure by the end there were no dailies left. This might be too tough below level 5 or so as only having 1 daily each for 4 significant encounters might stretch the party thin.
We run in Map Tool online, so there's not really time for more than two or so good encounters in a session (turns just take a bit longer, in my experience).
Good advice all around, here -- I'd sort of dismissed minions, since swordmages have some burst, but I suppose that it's pretty short range and not that much more useful than any other non-controller's. I'll give it a go tonight!
We run fantasy grounds which is similar to map tool. We also usually only get 2 or at most 3 encounters in a 3-4 hour session. part of this is do to roleplay and part due to trying to uncover the clues to the various plot lines we have going on.
As mentioned, this makes things much easier on the party. If you're sticking to one session per extended rest (as opposed to starting your next session right after the first one ended in game-time), you'll need higher-level encounters.
This goes back to my dungeon encounters vs wilderness/overland travel encounters. If you put an N+4 after 2 N+2's the party might be overwhelmed depending on the resources expended in the first two encounters.
