The difference between a "minion" and "just a really low-level monster," is that minions have the similar attacks and defenses to other "on-level" foes ... but they only have 1 hit point. In short, they're made of paper.
Sounds good in theory; in practice, minions don't usually last past the first round. The party wizard tosses out his AoE attack, and the minions are all gone -- very often without even having had a turn.
But! The idea is still worth trying to salvage. So today for my Giants game, I started putting together a usable "Minion" NPC class for 3.5. What I came up with is pretty simple:
HD: 1 hp/level.
Fort/Ref/Will: +1/2 level, rounded down.
Class AC Bonus: +1/2 level, rounded down. (Does not stack with worn armor, but does stack with natural armor.)
Proficiencies: All Simple and Martial Weapons; Light, Medium, Heavy Armor, and Shields.
Skill Points: 2 + Int mod/level (8 + Int mod at 1st level)
Class Skills: Balance, Climb, Hide, Intimidate, Jump, Listen, Move Silently, Ride, Search, Spot, Swim, Tumble
Special: Minion weapons all do damage as if they are size Medium or the minion's actual size, whichever is smaller.
CR: Total levels -4 or CR 1/4, whichever is higher
It's sort of a half-class, half-template, as you replace all of a given critter's hit dice or levels with minion levels, rather than stacking the Minion levels on top of their hit dice as you would with another class. For example, an ogre normally starts with four hit dice; if you wanted to make an 8th level ogre warrior, they'd have 4d8 (humanoid) + 4d8 (class) ... which would give them a huge pile of hit points that they really don't need if they're going to be cannon fodder. So instead, you strip out all their racial hit dice (and associated feats, stat raises, etc.) and just give them 8 levels of minion. The result looks something like this:
CR 4, CE Large Giant Minion 8
Init +0; Spd 25
Senses Darkvision (Ex): 60 ft., Low-light Vision (Ex) Listen +5, Spot +6
AC 17 (FF 17, Touch 9)
hp 24 (HD 8+16)
Saves Fort +8, Ref +6, Will +6
Atk +12/+7 base melee, +7/+2 base ranged;
Grapple +17; Face 10'x10'; Reach 10'
Melee +13/+8 Greatclub 1d10+7
Ranged +7/+2 Javelin 1d6+5
Abilities STR 20/+5, DEX 10/+0, CON 15/+2, INT 6/-2, WIS 10/+0, CHA 7/-2
Feats Armor Proficiency: light, medium, heavy; Cleave, Power Attack, Shield Proficiency, Simple Weapon Proficiency, Weapon Focus: Greatclub
Skills Climb +2, Jump +2, Listen +5, Spot +6
Possessions Greatclub; Javelin; Hide Armor, 3d6 gp
Note that this particular minion is intended to go into an 8th-level(ish) encounter, but is only CR 4. That's partly due to the wonky math of 3.x encounter balancing. With only 24 hit points, this would be a lousy foe to throw solo at four 4th-level characters. They'd chew him up, and it's not what he's intended for.
But the 3.x CR/EL system has built into it the idea that if one creature is "EL," then two of the same creature is "EL+2," and four is "EL+4." Thus, if you want the equivalent of one CR 8 foe, you have four CL 4 minions. It's wonky, but the 3.x CR/EL system has always been wonky.
Note also that he still comes out with 24 hit points. That's not enough to survive a big ol' fireball to the face, but it's probably enough to survive if he makes his saving throw. Similarly, it's probably too high for the fighter to one-punch him at 8th level unless the fighter is really tricked-out or manages to crit. I haven't decided if that's a bug or a feature. My original idea was that the minion would get 1 hit point per level and then his Con bonus once ... which in the case of the ogre minion here would give him 10 hit points total, definitely in one-punch range for just about any 8th level character no matter what they're doing. But that seemed too low, leading to the same problem with 4E minions, i.e., that they die before they even get a chance to go.
Another option would be for them to get 1/2 a hit point plus their Con bonus per level. For the ogre here that would still put him at 20 hit points, which makes him a two-hitter for the average 8th level fighter (which might be feasible with a full attack at +9/+4 or so). Finally, there's the possibility that they only get their Con bonus/level and no hit points for the class, with a minimum of 1 hp/level for those sickly minions with a Con of 11 or lower, and possibly a maximum of 4 hp/level for those tankety minions with Con 19+. That would put our ogre here at 16 hp ... which an 8th level fighter might be able to do on a really good round, especially with Power Attack, but isn't so low that he might as well not roll the dice at all.
Hmm. Must ponder!