And as neat an idea as "monsters with class levels" is on paper, I think 4E may be in the right here. In the "OMG Giants!" game, the heroes are going to be going up against a small army of drow who are assailing a dwarven outpost, and so I spent last night statting up a bunch of different drow types, particularly "warrior," "elite warrior," and "arcane guard," and the more I had to muck around with class levels, the more of an unnecessary-pain it got to be.
The arcane guard was the worst of the lot. The basic idea of a drow arcane guard is that he's basically a fighter but with a few neat little arcane trips up his sleeve. So to get that, going by the book, the obvious route is "eldritch knight." But to qualify for the EK, you need to be able to cast 3rd level spells, meaning you have to be a 5th level or higher wizard, but then he can't hit anything in melee, so then when you start adding on enough fighter levels to make him able to survive in combat he starts blowing out the intended CR, so maybe switch to warrior, but he's also got all these superfluous class abilities (like a familiar for instance) ... bah. This is already been too much work for a foe whose basic job is to show up, pull a few neat tricks, and then get killed.
Y'know what would be a lot easier? Looking up "humanoid" on the creature hit dice by CR table and building the arcane guard as simply an X HD humanoid with a handful of cherry-picked spell-like abilities. In fact, that can be done super-fast:
Target CR is 8, so 12 hit dice, which gives him +9 BAB ... make his Will saves good so Fort +4, Ref +4, Will +8 ... six feats ... 24+(Int bonus x 12) skill points. Add the drow racial features, give him some ability scores, gear, pick a handful of spells for his signature tricky moves ... done.
Drow Arcane Guard (CR 8 / XP 4,800)
CE Medium humanoid (elf)
Init +3; Senses darkvision 120 ft.; Perception +14
AC 19, touch 13, flat-footed 17 (+6 armor, +2 Dex, +1 deflection)
hp 78 (12d8+24)
Fort +6, Ref +10, Will +8; +2 vs. enchantment
Immune sleep; SR 18
Weaknesses light blindness
Speed 30 ft.
Melee +1 spiked chain +13/+8 (2d4+3 plus spell channel)
Ranged hand crossbow +12/+7 (1d4/19–20 plus poison)
Spell-Like Abilities (CL 12th) 1/day-- dancing lights, darkness, faerie fire, haste, lightning bolt (10d6); 3/day-- blur, invisibility, spider climb, ray of enfeeblement, shocking grasp (5d6)
Abilities Str 14, Dex 16, Con 14, Int 13, Wis 10, Cha 12
Base Atk +9/+3; CMB +12 (+16 to trip); CMD 25 (27 vs. trip)
Feats Combat Expertise, Greater Trip, Improved Trip, Quickened Spell-Like Ability (shocking grasp), Weapon Finesse, Weapon Focus (Spiked Chain)
Skills Acrobatics +15, Perception +14*, Spellcraft +13, Stealth +15
*Racial Modifier +2 Perception
Languages Elven, Undercommon
Possessions +1 spiked chain, +2 chain shirt, ring of protection +1, hand crossbow, 2x potion cure moderate wounds, d% gold pieces
Poison Use (Ex): Drow are skilled in the use of poison and never risk accidentally poisoning themselves. Drow favor an insidious toxin that causes its victims to lapse into unconsciousness--this poison allows drow to capture slaves with great ease. (Drow Poison—injury; save Fort DC 13; frequency 1/minute for 2 minutes; initial effect unconsciousness for 1 minute; secondary effect unconsciousness for 2d4 hours; cure 1 save.)
Spell Channeling (Ex): Once per round, if the arcane guard hits with a melee weapon, he may channel a quickened spell-like ability through his weapon against the same target as a swift action that does not provoke an attack of opportunity.
There you go ... a guy who runs up, trips you with his spiked chain (then hitting you with it for damage as an attack of opportunity if you fall down thanks to the Greater Trip feat), zaps you with a shocking grasp through the chain as a touch attack while you're down, then on the next round turns invisible and moves around to get into position to do it again. Pure cheese, and exactly the kind of thing a drow skirmisher should be doing. Since he's got spell-like abilities instead of spells I don't have to mess around with armor spell failure chances (or burning feats to counter them), nor do I have to bother with things like spellbooks, familiars, or sorcerous bloodlines.
As easy as it was to just build him as a whole new monster instead of trying to muck around levelling up a garden-variety drow, in 4E it would be even easier ... take an 8th level skirmisher, give it drow racial abilities, add trip and invisibility as at-will powers and the shocking zap through the chain as a recharge 5-6 power, and he'd be done. So in that regard, for all the hate I hold dear against 4E for what it did to player characters, it is actually much better as a GM toolkit. As it is, with the build above I ignored things like what are "class skills" for humanoids and just gave him four skills with +12 ranks each.
Honestly, if I found a way to have 3.x/Pathfinder player characters in the 4E ruleset, I expect I'd jump on and not look back.