Let me get this out of the way: I love minions. They’re one of my favorite 4e mechanics, for a variety of reasons. I like that I can attack the party with a horde of enemies and not overwhelm them. I like that I can easily model weak enemies that are easy to kill, and put them alongside tougher enemies. I also like that I have a way to build an encounter that allows the players to feel like badasses, as they hew through enemy after enemy, leaving a trail of destruction in their wake.
There are, however, other ways to use minions. Minions, when properly designed, can be used to create en element of the encounter that can be a significant consideration, or even a significant threat. The trick is to find a mechanic or an idea that will force your players to alter their tactics in response, and then build your minions around that mechanic or idea, allowing them to exploit or even drive it. I’m a firm believer that it’s easier to tell someone how to do something by showing them, so I’m going to insert a few minions of my own as examples, and I’ll talk about each one and what its purpose on the battlefield is.
A standard goblin stripped of any weapons and strapped to a cask of gunpowder, the exploding goblin is a creature that is a nuisance when it’s alive, but deadly when you kill it. The creature becomes particularly deadly when you’re surrounded by them, as one goblin killed could result in a cascading explosion that does massive amounts of damage. Under normal circumstances, bursts and blasts are great for taking out large numbers of minions, and minions die within the first two rounds of combat. With these minions, the question becomes: do we try to take a bunch of them out at once, and risk being damaged by the ensuing explosion, or leave them around to hamper us and crowd around us? In the latter case, there’s the risk that the enemy will exploit their exploding nature, killing one or two of them once they’ve crowded around the group.
The phase beast is a large, dog-like creature native to the Feywild. The idea behind the phase beast is to create a minion that doesn’t deal any direct damage (though it can move people into environmental hazards), but causes all sorts of headaches for the PCs while it’s around. Being teleported and dazed can be a real nuisance, meaning that these creatures, even though they don’t pose much of a direct threat, are likely to be targets early. The problem is, once a phase beast is killed, it tears a hole in space that lasts for the entire encounter. This modifies the battlefield, creating a 3×3 area that is difficult to get through. Artillery, controllers, and other squishy, ranged monsters can hide behind these rifts, and because the DM gets to choose the destination of the teleport effect, they can even use them for rapid transportation across the battlefield.
Use effects like this sparingly; an encounter with six phase beasts could quickly create a battlefield that is extremely difficult to navigate (if not impossible), turning the encounter into a frustrating grind. Creatures like this can spice up a fight, causing players to make interesting decisions during the fight, but two or three is generally plenty. Alternatively, you could use a large number of them, but tone down their Spacial Rift power to affect a single square, or to end after a single turn. As an aside, using minions like this can make wizards happy that they took powers like Dispel Magic.
Tiny motes of white light that are actual fragments of a living prophecy, a prophecy mote’s only goal is to impart knowledge of the prophecy to other creatures. I used these creatures in an encounter a while back, and they require the encounter to be built around them in order to avoid a total party kill. The thing to remember about these creatures is that, no matter how many you kill, more are always coming. Because of this, the party will eventually get overwhelmed by them unless you give them a way to stop the prophecy motes from spawning, as well as to get rid of the ones that are already there. In the encounter I ran, the party had to, one by one, examine a prophecy mark on the ground and learn their role in the prophecy; only then would the motes leave them alone.
Using mechanics like this can turn a combat encounter into a puzzle. Unlike many puzzles, however, this one is structured in rounds like a combat, and has an escalating consequence for not solving the puzzle. Creatures like the prophecy mote are not suitable for most encounters, but work well when they are the focus of some sort of puzzle.
There are very few minions that mark, and with good reason: minions tend to crumple pretty easily, and tend not to hit very hard. However, minions that mark can be used to create interesting decisions for players, particularly if those minions punish severely for defying the mark. The best way to use a minion like the eladrin guard is to have him target strikers; to help him achieve this end, the eladrin guard is a bit more mobile than most soldiers, able to teleport past the defenders to get at the strikers in the back ranks, as well as pursue those strikers across the battlefield. This gives the striker an interesting choice: do I use my considerable damage to hurt a regular monster, thus incurring a penalty and an attack, or do I waste my damage on a minion? Unlike other soldiers, these minions work well in larger groups, so that they can spread their marks around and force other interesting decisions: namely, whose marks are most important to eliminate?