I hate Mummy Lords. There's a reason I hate them. On the surface it might seem that I hate them because this evening my group ended up fighting one for roughly two and a half hours, and we're now all spent in terms of healing surges, daily powers, and hit points- to say nothing of our prospects on surviving the Mummy Rot we all contracted, or the next encounter. No, the reason is that the Mummy Lord in question was a level 10 Elite Controller, and we were all level 7.
I've seen this far too often- and I've done it myself, once. You look at the way that encounters are built, check out that xp budget, and realize that, hey, you can have your group of level 1 PCs go up against a single level 6 elite monster. That's fair, right? It's within your xp budget after all.
The answer is, no- no it isn't. And the reason that it isn't is encapsulated in a small, seemingly casual warning in the DMG: stick close to your PCs level.
You see, what folks don't seem to realize is that xp value is only half the story. A monster's level is important too, and while you have some wiggle room, there is an upper limit to what's viable. There's a certain point where your group is going to end up frustrated, drained, and bored of the encounter- and your monster (or monsters) are likely not even bloodied yet.
Let's take that Mummy Lord as an example. It's a Controller, so it's not particularly suited to melee- at least on its face. However, with its lowest defense at 20, your players need a +10 to hit 50% of the time, and need to be able to target that defense (REF, in this case). Against any other defense, the target number goes up. This can already be problematic; my own character has a +10 to hit: +2 for magical enhancement, +2 for proficiency bonus, +3 for STR, and +3 for level. And I have the option to target REF with some of my powers. I'm not optimized, but I'm no slouch.
Now add on to that the Regeneration 10 that requires radiant damage to turn off, and the +1 AC that the Mummy Lord enjoys with his basic melee attack. Suddenly things are looking a heck of a lot worse, if only due to the fact that you need to deal at least 10 damage to this thing each round just to keep it injured. The fact it can boost its defenses while hitting you makes it worse, because if you're my character, that boost to AC means my prospects of hitting have now dropped to 25%. And again, I'm no slouch.
Tack on the fact this thing has over 150 hit points and it becomes interminable. With a group of six, you can expect roughly 1.25 hits per round versus AC. If we assume that 0.25 is the group's Wizard spamming Magic Missile, you can round that up to two. On average, a hit will do around 10 damage- or with MM, and again using my group, you're hitting for 18 per round if the entire team gangs up. That means you're hitting for a net 8 per round. If you can consistently smack this guy every single round like this, you've got a 20 round fight on your hands. And that's before he hits back, and assuming you're lenient and ignore the words 'Second Wind' on the Mummy Lord's stat card.
I ask you- does this sound like fun? It isn't. It's a long, frustrating slog through a tremendous amount of hit points while constantly burning healing surges, encounter powers, and other resources to stay in the fight. Change it to an Elite Soldier, and you might as well just have your party throw their dice at the wall and call it a night.
So I'm asking you- begging, if you prefer. Keep your monsters close to your party's level when you build your encounters. Think of the Mummy Lords.