Dragonblade
Adventurer
There's nothing in the RAW that states you can't do exactly what they do and have to pull your punches. Nothing keeps players on their toes like the actual threat of dying. Those have always been my most memorable moments as both a DM and a Player.
Right. I don't mean to sound like I'm taking a shot at 3e, but I often found that encounter design was more art than science and I had to constantly be prepared to fudge in favor of the PCs if an encounter was too hard. 4e does a much better job at balancing encounter design, so that I as DM don't have to pull punches.
Once I have established and designed the encounter, my referee/storyteller hat comes off and my adversarial/monster hat comes on. I will now do my utmost to crush the players, to see them driven before me, and to hear the lamentations of their women.

That means I don't just have my brutes march up to the defenders and stop there, hacking away. Or let PC controllers sit back and attack me from the rear ranks with impunity. I go for the throat.
I immediately focus fire on their controllers and their leaders. I only consider OAs, and marks when it could potentially stop my monster's movement in getting to those leaders and controllers behind the defenders. If a lesser monster can sacrifice itself to put a stronger one in a better position to bring the pain, I do it.
I don't run my monsters out of fear of drawing an OA, or stop at the defender because I need to somehow RP that they are a threat. Mark me, hit me with that OA, because my monsters are coming for you!
Its not my job as the monster guy to be timid and reactive. My job is to defeat the players, and I do that as efficiently and brutally as I can. Its the players job to stop me. Its up to the strikers to position themselves for max damage. Its up to the defenders to use their marks to protect their allies. Its up to the leaders to heal themselves and their allies before I take them down. And its up to the controllers to seize control of the battlefield from my own monster controllers!
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