Are your PCs designed properly? As in, do they use reasonable point buy? Are they going out of their way to boost AC? (My players almost all boost AC into the stratosphere, and the answer is to modify monsters that you're using. So, there's no reason a fighter-type NPC or monster using a warhammer can't target Fort rather than AC, just apply the -2 penalty to hit. A monster with a tripping attack, such as a wolf, might target Reflex rather than AC, again just apply the -2 penalty to hit. A monster with a spear might also target Reflex instead of AC, just apply the -2 penalty to hit.)
Players are consistently denying actions to my monsters, sliding them around and knocking them prone like the Three Stooges. I can’t threaten “squishy” targets at all due to marks, auras, etc., from the defenders. It’s rare that I ever do enough damage to bloody even a single PC in a fight. I up the number of monsters and even fudge die rolls to give myself a fighting chance. I had only a single PC fatality in the most recent Lair Assault – and it was everyone’s first run through.
My PCs do the control thing too (they have two leaders subspecced as controllers, and one, a barbarian, has a few powers which can push opponents). I've literally never had an NPC soldier get an attack against someone violating a mark, as they're always getting shoved away from their charge, or their charge is getting slid away from them. That's not a problem to me though. PCs going out of their way to avoid bonus attacks means the NPC soldiers are doing their job by forcing the PCs to react.
Too much control? Return the favor. Controller monsters exist for a reason. Your PC wizard (if you have one) can use Icy Rays to immobilize two opponents for one turn. A lizardman "shaman" druid can use Swamp's Grasp to grab multiple PCs until they save against that. In H2 (I know this is before the monster math, but you can adjust) there's a duergar "theurge" who can summon brimstone as a recharge or encounter ability - it does AoE fire damage and knocks the victim prone - and since being knocked considered a minor effect, it's not taking any damage penalty. Is there any reason you can't do the same? My own PCs complain that they found dazing to be a weak condition against monsters, but it's so much stronger against them (especially in a leader-heavy party; being unable to heal and attack in one round is a pain for them).
Last session that I ran, I had an eladrin wizard doing exactly that to the PCs, along with a half-giant who used a spiked chain, and a couple of crossbowmen. At one point, the party's best healer had moved more than 5 squares away from the rest of the PCs and then gotten immobilized, and most of the PCs were prone due to constant tripping. I wasn't even aware of the tactical possibility here, it just came up by accident: the cleric was too far away to use healing, which was bad because the other healer had just gotten chained into unconsciousness
There are numerous ways of avoiding defender blocking. The easiest is to use teleporting. Even if marked (-2 to hit, possibly drawing damage) marking attacks rarely do enough to threaten monsters. Only the psychic warrior is that good at it, and setting up the marks is difficult. In other words, eladrin rogue NPCs are deadly, especially if they're using Tactical Trick to give them constant combat advantage against everything all the time.
Artillery, the next easiest, should do high damage, and many have damage bonuses in certain situations (when they can spend a minor action to boost damage, when opponents don't have cover, etc). Even if they're being paladin marked, they can either absorb the damage, or use their additional +2 bonus to hit with ranged/area attacks to target the high AC paladin, or use AoE attacks that don't target AC to hit both the paladin and a squishy (and incidentally probably target the paladin's weakest defense, and still get the +2 bonus on top of that).
Another is to use skirmishers. Depending on the fighter build (you said "aura", so I'm thinking knight) this can be quite easy. PH1 fighters have Combat Challenge. The upside; hitting a marked enemy makes it stop moving (murder on skirmishers). The downside; the fighter has to have a chance to attack. Many skirmishers can simply shift past the fighter and attack a squishy. The fighter cannot lock down the skirmisher until it's turn (when it can finally make an attack). This does assume the fighter didn't attack the skirmisher first, but if they didn't, then who is protecting the squishy? On the other hand, you may have a knight. In that case, use two skirmishers. Just be sure to avoid the 1 square aura. Said knight can only stop the attackers if they've got Staggering Hammer, and even then, only a few times per encounter. Otherwise, at best they can slow the skirmisher.
Note that some skirmishers have abilities that give AC or defense bonuses when moving or against opportunity attacks (the knight's bonus attack is an opportunity attack, so I believe that would apply).
I’m not saying I want to be a Killer DM, but I would like that the players feel threatened and challenged and aren’t getting bored with the game.
I've found some encounters can go more easily than expected, and others can be deadlier than planned. With experience, you'll have a better idea of what the PCs find scary. Apparently it's not Lair Assault. I've never had the luck to play through that, though I hear it's pretty deadly.
Let's see. The Stooges comment was in regards to Lair Assault - The Talon of Umberlee. The characters were 8th level. The sliding was caused by a controller and a leader working in tandem.
How many monsters could they tag team simultaneously like that?
As DM, after I see that my monsters are just swinging at the air, unable to hit the defender except on a Natural 20 and even then just taking off a measly 10% of his hp, I'm prone to want to pack up, forget about the whole thing, and grab a beer. Particularly when the players are high-fiving and making fun of the ineptitude of the challenge.
As mentioned by other posters, something is fishy about this. (Also, use leader monsters and auras/fantastic terrain.)
