So I have been wondering about how this would work.
Let's say a 7th level Fighter with the Whirlwind Attack tree of feats and Lunge, and his cleric ally are surrounded by a diminutive swarm. They take their damage, but make their saves and are unaffected, then his cleric ally with the Law Domain hits him with the Touch of Law ability and rushes out of the swarm.
Whirlwind Attack (Combat)
You can strike out at every foe within reach.
Prerequisites: Dex 13, Int 13, Combat Expertise, Dodge, Mobility, Spring Attack, base attack bonus +4.
Benefit: When you use the full-attack action, you can give up your regular attacks and instead make one melee attack at your highest base attack bonus against each opponent within reach. You must make a separate attack roll against each opponent.
When you use the Whirlwind Attack feat, you also forfeit any bonus or extra attacks granted by other feats, spells, or abilities.
Touch of Law (Sp): You can touch a willing creature as a standard action, infusing it with the power of divine order and allowing it to treat all attack rolls, skill checks, ability checks, and saving throws for 1 round as if the natural d20 roll resulted in an 11. You can use this ability a number of times per day equal to 3 + your Wisdom modifier.
Supposing that this Fighter has an enhanced 20 Strength, has weapon focus & specialization with his +1 longsword, is 2 handing it, along with weapon training; we are looking at a +15 attack bonus and 1d8+11 damage. Since he can take 11 against every enemy this grants him a 26 attack.
On the Fighter's turn, he utilizes Whirlwind attack and Lunge to reach the entirety of the swarm. But instead of attacking the swarm, he targets each individual creature in the swarm. Even their minimum damage would normally be able to defeat a single creature of the swarm.
Normally a fine or diminutive swarm is immune to weapon damage. But would it be in this case?
Would you let this work, since it took such an unusual and specific set of circumstances to get here?
Would you rule it that the swarm simply loses its immunity and is damaged normally?
In the case that the swarm would lose its immunity and take damage, would this be considered a area of effect attack at that point?
Or would you simply not allow this to work at all, since it's still weapon damage?
i think the swarm is considered one opponent. you can not decide to treat it as multiple. you only get one attack on it the same way you only get one attack of opportunity when it enter your space (And not one per creature in the swarm).
so in this case :
'A swarm of Diminutive creatures consists of 1,500 nonflying creatures or 5,000 flying creatures" you do not get 1,500-5000 attacks. you get only one. and if it's a weapon attack it deal no damage to the SWARM ("A swarm composed of Fine or Diminutive creatures is immune to all weapon damage").
also:
"..A swarm is immune to any spell or effect that targets a specific number of creatures .."
one attack per swarm member is also a specific number. the same as one attack per level would be (say from telekinesis).