I just ran the finale of my 4e campaign, which capped off at just 11th level.
**** this game. **** it and its ridiculous enemy-crushing combos. The final boss was a 15th level elite with seven level 11 allies who showed up first to wear the party down. Then once folks were sufficiently bloodied, the big bad arrived, split the party in two by creating a wall, then hurled one PC across the room and proned him with serious damage.
The prone PC (a hexblade) stood and charged, hit and dazed and weakened the baddie, then used an action point to deal more damage and restrain the enemy. The only other PC in the room (a druid) also charged, hit and knocked prone, action pointed, then hit again and imposed a penalty to attack rolls. He was now bloodied.
The big bad only had one action, so he attacked from prone, and indeed managed to hit, but only did half damage. And for his trouble he provoked an immediate reaction attack. I had him action point and attack again, which could have dropped a PC if the druid hadn't used an interrupt to grant the hexblade a bonus to defenses.
The two PCs attacked again. The big bad was dazed again. I spontaneously gave the big bad another 100 hit points so he could actually do something. The other two PCs managed to circumvent the wall and came within range, then action pointed and weakened the big bad again, leaving him with only 50 of the bonus HP left.
I Bull-Sh***ed a power for the villain where he cracked the ceiling of the whole dungeon, so that only his own telekinesis was holding it up, and if they killed him it would fall on all of them. He had to action point to stand up.
The PCs (quite cleverly, to be honest), hit him with some minor attacks that gave him a variety of "ongoing damage," then opened a short-range teleportal and ran through it. At the start of the villain's turn, the ongoing damage did him in, and he only crushed himself.
Satisfying for the players, sure, but only because I wildly broke the rules. PC powers -- in particular their ability to shut down enemies with daze, weaken, and stuff like "slide and knock prone" -- make it impossible to challenge the players with even a level +4 encounter. And that encounter involved a lot of enemies which slowed down play.
TL;DR -- Get rid of dazed and weakened, and make the ability to shut down enemies much rarer.