One of my concerns at Paragon and Epic is that it is difficult with the RAW to challenge the PCs. Certainly there isn't as many mechanics to scare the players themselves (like level drain, instant death, etc).
However, I've noted that massive amounts of damage still does the trick (a hydra with some extra heads and spending an action point does wonders).
So here is an option you might want to try: the Brutal Elite.
Like normal Elite monsters, these count for two monsters. Some Elites (especially those more recently created) have multiple attacks compared to the normal version. However, in this case instead of adding another attack (which was usually just the basic attack anyways), you double the damage. For example, a monster that does 2d8+5 damage now does 4d8+10. Ongoing damage is also doubled.
One aspect I am considering is not doubling the hit points. This would make the monster very dangerous, but easier to take down. Effectively it would be like doing the double damage/half hp house rule that many people have done.