I'm still testing this out so if you're going to use it, be wary and use your common sense and good judgement where necessary. The math is roughly tailored to 3.x D&D, so if you use a different edition, you'll have to tweak that, but you can steal the structure.
Step 1: Figure out the average level of the party. Let's call that [Bonus].
Step 2: Write down the following values "Weak [Bonus-1]", "Normal [Bonus + 1/4Bonus]", "Strong [Bonus + 1/2 Bonus]"
Step 3: Total all the maximum Hit Point values of all the characters in the party. Let's call that [Budget].
Step 4: Divide your [Budget] by 4. That's your [Damage Budget]
Step 1: Distribute your [Budget] among a number of monsters, the points spent represent their HP.*
Step 2: Give each individual monster a damage output, spending your [Damage Budget]. (a d4 is worth 2, a d6 3 etc., otherwise they deal 1 dmg minimum)
Step 2: assign the Weak, Normal and Strong bonuses to each individual creature's AC (starts at 10), HP, Attack (Melee & Ranged), Damage, Saves & optionally Special Ability DC (starts at 9) or Special Exploit.**
*If it's just one creature you might want to consider giving it an extra 1/4 hp. Also, I recommend against giving creatures less than 4HP (for budgetary purposes).
** When assigning the bonuses you can follow these guidelines:
Sniper: Weak AC, Weak HP, Weak Melee Attack & Damage, Strong Ranged Attack & Damage, Strong Reflex, Weak Fortitude, Normal Will.
Dominator: Weak AC, Weak HP, Weak Attack & Damage, Strong Will, Normal Reflex, Weak Fortitude. Strong primary power, Weak secondary power.
Pick two powers: Area impair visibility, Area impair movement, Target Debuff, Area damage, Target teleport, Target summon.
Guardian: Strong AC, Strong HP, Normal Attack & Damage, Strong Fortitude, Normal Reflex & Will.
Savage: Weak AC, Strong HP, Strong Melee Attack & Damage, Strong Fortitude, Weak Reflex & Will.
Sneak: Normal AC, Normal HP, Normal Attack & Damage, Weak Fortitude, Strong Reflex, Normal Will, Strong Exploit: extra Attack & Damage.
Chief: Normal AC, Normal HP, Normal Attack & Damage, Normal Fortitude & Reflex, Strong Will. Normal Exploit: Target buff ally, Weak Exploit: Area buff allies.
I need to make a spreadsheet for this.