I have found that in a balanced party, I have to have about three different types of monsters to avoid having the encounter fully in the control of the PCs.
If I ran simpler encounters I could probably get away with adding 50% to the monster damage without the players noticing. (This is at level 8). All the monsters would be hitting the stupid dwarf in fullplate with a bunch of temp hp, second wind and two heals from the bard (healing surge+6+1d6 and 6 temp hp). The Invoker would probably have a field day AoE nuking without any fears... etc...
If you look at the base damage on the monsters they go from around 3 average hits to around 5 average hits to take down a PC. It must be said that higher level monsters have some nice limited expression damage boosts, but in the long run, it just takes more hits to take down a character at level 8 than it did at level 1.
Take the example above, the dwarf needs 5 hits to get down to 0% hp, or say 10 attacks. The healer and second wind increases his hp by about 125%, so that increases the number of needed attacks to about 23. If 5 monsters where attacking him for 5 consecutive rounds, he would go down. This of course never happens. Usually, one monsters dies each round except the first... In addition, he generates temp hp himself so he could probably last 6-7 rounds...
This is with the Normal (High) damage expression from the DMG, page 42, usually only brutes have this high damage or 5 of the damage is ongoing damage.
There are some limited damage expressions (high) that changes this somewhat, but either it's on a recharge power, it's hard to do (skirmish/sneak attack), or the damage is plain blocked by PC abilities (immediate interrupt powers, like the level 3 ranger power that gives the attack a penalty of about 6. - The ranger needs to hit, but he uses his elf re-roll here, so it works about 75-85% of the time depending on the monster, it's usually not used against soldiers, so 85% is probably the right number).
What I am gonna do to get combat more challenging is to make combat less predictable. Instead of running combats with 5 monsters of the characters level, I will start with a ordinary encounter of about level+0 and have some monsters joining the fight about round 3, when the characters usually gains control. I can't do this every fight, but it will increase the tension because the players never know if the encounter DOES turn dangerous, even if it looks like a walk in the park.
I will probably go from avg 4.5 encounters per day to 3 encounters, giving about the same amount of XP, but keeping the players on their toes.
I LOVE the way a DM has so much more control over the dangers of combat in 4E, but I do think I have to re-learn what makes encounters full of tension.
I will NEVER run a Solo monster alone btw, I have done 2-3 single SOLO mob fights and it got SO predictable I nearly fell asleep.
Since then I have tried to run a Solo + 4-5 artillery, but that was overkill again. It basically left two PC's to handle a complete encounter by themselves.... (I cut the extra mob hp in half to fix it...)
The second time I had the solo dragon awaken undead with its breath weapon and it was a lot of fun both for me and the players. New monsters appearing during the fight worked really well.
