I believe there may be a Mob template - I apologize I cannot for the life of me remember where it is. However, if you're looking for a way to group them together as a single entity, strategy game style, then my main suggestion would be to assign them as a templated unit; for every X HP damage done, a "unit" falls off; you can roll attack & damage for the unit, and it will deal damage to a single target.
For example:
4 Dire Worg Skeletons (CR 3 I believe) are occupying a 10x10 sq, for purposes of being subject to AoE effects. We now assign a template to them: Worg Pack.
Worg Pack: Take the average tally of HP, and grant the Pack a bonus to hit equal to the total number of members divided by two. We have four members, they're more likely to hit, we grant the pack a +2 (in this example). Because a Worg Pack may have a total STR bonus of +12 to damage, and we don't want to kill the party member outright, you assign the STR bonus once, unless you deal critical damage, in which case you assign it twice (once for each multiplier, in this case, two).
Base Attack: 3 + 2 (members) + 3 STR = 8 BAB. This gives 4 Worgs a pretty good chance to nail someone. There is always a minimum bonus of +1 to hit so long as a pack exists (two members or more). A Worg Pack (or any template like this one) follows the same rules as anything else. You can make two arguments here.
Tougher: Only the front-rank members can make an attack as a standard action. You would thus roll a normal attack, +1 instead of +2 (members attacking/2) and deal damage (1d6 per member, plus STR bonus once) and deal damage normally. This "simulates" the way the system is designed, while keeping your template effective.
RAW: You may only execute one attack, per the base creature, when the Pack moves. That's a DM call, I tend to use both front attackers when I've used this rule in other campaigns to do exactly what you're trying to do.
When you deal damage, (d6 + STR) you'll roll the d6 for each pack member in question. Total damage is dealt to the target normally.
Full Attack: Roll your attack & deal damage at full bonuses and with all active units within the pack. Yes, this will hurt someone very badly. A Pack counts as using a two-handed weapon for purposes of determining damage on a Power Attack (when applicable).
Size: A pack counts as the size appropriate to the number of squares it occupies with its current members.
Total Attack: A pack may bring a total number of members against a single character, but never more members than would normally be allowed to attack a surrounded character (8). A Pack may divide its attacks among multiple targets using a Full Attack if it so chooses, attacking any target within range.
Spell Effects/Saves - the Pack makes saving throws as a normal creature of its type, with a bonus for (members/2). This bonus has a minimum value of 1 (so long as there's a pack, there's a bonus). Otherwise, like the STR bonus, any stat modifiers to saves are only applied once.
Casting Spells - a Pack may only cast one spell/use one SLA etc. per round.
Reducing a Pack - Once you've whittled a pack down, it isn't nearly as effective; it will eventually simply be two creatures attacking in tandem, dealing damage in tandem on a FAA. Damage to a pack is dealt to the pack as a template, regardless of damage type. Once the appropriate increment of damage is done (1/4th of a 4 member pack) a 'member' of the pack goes down, and the stats are adjusted.
This is a little complicated, but it may give you the effect you're looking for. This doesn't cover mobs/packs composed of multiple creature types; that would get clunkier. And, this is a hybridization of rules I've used before; if you can find the RAW for mobs, that might be much simpler than my homebrewed rules.
So what's left after the fireballs must be destroyed in melee.
