There have been some very cool posts lately about using extremely dangerous and overpowered monsters in neo-classical and modern gaming lately. I'd like to sound a cautionary note. First, understand that I agree with just about everything I've read about these sorts of encounters and how cool they can be. However, before you include them in your game, you have to keep in mind just how dangerous, and anti-climactic, they can be.
The alpha strike is a tried-and-true tactic from the earliest days of D&D. The name is stolen from Star Trek wargaming. In the game Starfleet Battles, a player launches an alpha strike by maneuvering into the optimal range (usually point-blank) and then cutting loose with every weapon on the ship. It perfectly recreates those moments in TV and movies where the ships crawl towards each other, and one is blasting, blasting, blasting while the camera shakes with every hit and someone is calling out in tones of escalating concern the worsening condition of the shields, the soundtrack rises to a tense pitch, and the captain yells, “FIRE!”
In D&D and similar games, the alpha strike usually doesn't involve that slow buildup. The PCs immediately unleash their most devastating attacks upon a single target. This makes sense, if you think about it, because a critter with only 1 hit point left is almost always as dangerous on it's turn to attack as a critter that still has all of its hit points. Focusing on one target at a time is usually the most efficient way to remove the monsters' offensive power.
Even in cases where the monsters do get weaker, players are encouraged to launch an alpha strike. The damage from most breath weapons in early versions of D&D is usually equal to that monster's current hit points. The quicker you whittle those away, the more likely you are to survive the blast. This is why, when you put together an encounter of a bunch of goblins and a hellhound, the players will likely ignore the goblins as much as possible until the hellhound is dead.
This can be annoying for the DM, because it's the reverse of a good cinematic fight. Instead of the danger ramping up as the players wade through the minions, you get a brief moment of tension, followed by a drawn-out mopping up phase. (You might be tempted to save your villain by declaring that he survives or even shrugs off this attack. But now the players are looking at a foe who hasn't been noticeably weakened and they've likely burned resources in their alpha strike that they can't get back without resting up. The players will probably retreat in this case, in order to come up with a better, more certain one-shot kill.)
Here's something else to keep in mind about alpha strikes: magic-users use 4-sided dice to roll their hit points, and usually 6-sided dice to roll damage. Your typical 6th level magic user has 15 hit points and her average fireball or lightning bolt will do 21 points of damage (10 if you roll your save). Magic-users are both scary and fragile. They usually end up on the receiving end of an alpha strike, because if you don't do unto them first, they will almost certainly do unto you.
(This also means that wizard duels in D&D-land are not drawn out battles of wits, or back-and-forth exchanges of spells. They look more like the gunslinger showdowns of the Wild West, where the fastest draw, er, I mean, incantation, usually wins.)
Keep this in mind when using magic-users in your adventures. The first time the players see how effective sleep is as a spell, it's going to make an impression on them. They absolutely do not want to go out to a coup de grace like those goblins in their first adventure. If they see a spell-slinger, they are going to pile on the hurt. This is why the guy in the pointy hat dies first, and usually with extreme prejudice.
Often, the best way to protect your magic-users, either as a player or a DM, is to make sure they don't look like magic-users. Obviously, once they start slinging spells, the jig is up, but by making the magic-users look innocuous, they might actually survive long enough to get off that first spell. Generally, the first and best magical protection that isn't armour goes the magic-users as well. They also usually carry at least one healing potion, since any hit they take is likely to be catastrophic. It's also common to give the magic-user a bodyguard of some sort, just in case somebody rushes in and tries to stick something sharp and pointy into his or her face.
Photo credits: Focal Intent, Benimoto, and seanmcgrath.