The PCs in my game are only 7th level and I still have a very difficult time challenging them.
I often put them into an adventure 1 to 2 levels higher than they are, just so that the encounters are not finished in 6 rounds with few resources expended.
As a suggestion, have more foes, but not minions, hidden in a very large not so well lit combat area. Have multiple initially disguised teleport traps where the PC heads in a direction and boom, he is on the other side of the map, thus forcing distance between the PCs. ~5% of the squares should be teleport traps and they should be easily recognizable (maybe it glows faintly) once that given trap is used (either as a sender or a receiver). Before that, it should take a very high Perception roll to find one. Have the receiving locations be another teleport trap and where a PC ends up is totally random.
If you throw in some difficult terrain, that would make it difficult for the PCs to regroup. By large, I mean something 30x30 or even larger if you can manage it (50x50 would be great, but that's hard to get on some tables). Something that takes a long time to move to the other side. The first PC that steps on a trap and ends up far away with a few monsters nearby will force the other PCs to minimally move towards him and boom, they too will start teleporting. If they use his square, they will go somewhere else. A creature cannot teleport to a square that already has a creature there.
So, if you often have 5 foes in a fight, make it 8 or 10 (one level less up to two levels higher) foes for this one. If none of them are minions and the players are expecting minions in such a large encounter, it will drive them crazy figuring out which are the minions for the first few rounds until they realize that there are no minions.
Enclose the area so that spell casting PCs cannot fly up out of harms way. Don't make the ceiling more than a few feet taller than the tallest creature and don't use creatures more than one size different. Put a lot of 2x2 or 3x3 pillars in the chamber so that PCs have a tough time spotting foes, using the diagonal distance is the same horizontal and vertical aspect of 4E, etc.
With regard to the teleporting, allow the foes to control the teleport when it occurs within 2 squares of a foe. For example, if the foe does not want to teleport himself, he does not. For a foe to get far away, he merely steps on a Teleport trap square and goes exactly to the target teleport trap square he wants. If a PC tries to get away, the foe as a free action (on the PC's turn) can prevent that teleport square from working. The PC has to move further away from the foe in order to find a teleport square, hence, it is difficult to do something like: attack with slide, teleport away. Instead, it becomes attack with slide, no teleport, use move action to get to a different teleport square if one can be found.
Use 4 different types of foes and do NOT follow a theme (i.e. not all fire creatures or not all undead). Mix it up. Typically the suggestion is to not have too many different types, but this should be a bit of an Epic battle where each foe has different abilities which assist those of other foes.
1) One elite controller. Not only will he harass the PCs, but since he is elite, their anti-controller tactics might work less successfully due to his double hit points and superior attacks. His job is to slide PCs onto teleport squares.
2) 2 to 3 Lurkers. Their job is to gang up on the squishier targets and then teleport away and gang up on someone else. Keep them close to each other for group support.
3) 2 to 3 Brutes. Their job is to just pound on PCs and damage them. When they get surrounded by PCs, they too should teleport away to fight some other PCs. If they can push, all the better.
4) 2 to 3 Artillery. Their job is to attack from long range and stay out of combat. Whenever a PC gets close enough to attack them, even a ranged PC like a Ranger, they move far far away. Granted PCs like Rangers have 20 normal range, but the Artillery should use the pillars as cover and the low light as concealment to increase their defenses. Total cover should be possible in some cases.
Try to find interesting foes like Death Titans that can heal themselves or have other unique and interesting abilities. Having a few foes that can slide PCs onto the teleport squares would be good too.
And, seal off the room (e.g. a solid steel door drops at the entrance or something) so that the PCs cannot just run away.
The entire idea of this encounter is to decrease the action economy of the PCs. It does it in three ways: 1) by splitting up the group, it forces the PCs to use up actions just to move closer to allies to heal or help them in other ways, 2) by having the NPCs control nearby teleport squares, it forces a waste of a move action for PCs to teleport away when they get separated and focus fired on, 3) it often concentrates fire on a single PC in order to take that PC down quicker, and 4) PCs should be so far apart that area effect PC all encounter buffs and such rarely help all of the PCs.
Don't allow the NPCs to get bogged down by PC Defenders or Controllers if possible. Even the Brutes should try to be extremely mobile most rounds. If an NPC is in melee with one or more PCs for more than 2 rounds, he is probably there too long unless he has a PC really on the ropes. Keep all of the NPCs often on the move.
The difficult aspect of this encounter is figuring out the location of the teleport squares. One way to help with this is to not allow a teleport trap within 3 squares of another one, that way you can visualize safe zones without worrying about every square. Another is to have some form of deterministic system mapped out ahead of time.
The cool thing is that the monsters can move wherever they want and since they control the teleport traps, a PC can move on the same square a monster just moved on and still teleport away. A PC can be attacking on a teleport square, not know it, and the foe just has the PC teleport away before the PC can attack, etc.
The key to this, though, is to ask yourself every single round: "What can I do here to make it harder for the PCs and easier for the monsters?". Don't get too wrapped up in the encounter that you forget to use the trap to the best of your ability. Slide PCs onto Teleport Traps and move monsters there as often as possible so that the encounter is constantly shifting and plans that PCs make are constantly thwarted. Don't let up. On the other hand, the PC Rogue or whomever should be able to disarm a given trap with maybe a 50% chance of success.
Final note: Allow for one random location to be out of the room. This allows the monsters to escape if they are super seriously damaged (player hate escaping monsters), and it allows the monsters to send a PC out of the room so that he has to waste time coming back in (use this infrequently or as a last resort because that is the monster escape hatch). However, if that random location does randomly come up and a nearby monster is not controlling that trap, let the PC be removed from the map. That will make for an interesting moment until/unless the PC moves back on that square to come back.
