[Varyon]

Aura seems like the way to go, provided it's legal to toss Area Effect on there. Swarms simply automatically attack anything they come in contact with (unless they choose not to; if they can make such a choice, you'll want to add Selective Area), and I believe Auras are basically just toggled on and off as free actions - my initial thought was Emanation, but that calls for an attack.

No Roll Required and No Active Defense are, I feel, inappropriate modifiers. For the first, the attack can only hit the hex the swarm is in (it's got only 1 yard of effect, and no range/reach), so there's no roll required anyway. For the second, an area effect can only be Dodged if you can get out of the area... and I see no reason why you couldn't do that against a swarm (granted, if the swarm still has movement left, it can just move to wherever you ended up after the Dodge and Drop).

If you want the swarm to be able to get past armor/clothing... I'm honestly not certain if Contact Agent is needed, if it has Infiltration. The former means the target suffers full effect immediately, while the latter has a built-in delay, during which time the foe could escape from the swarm, or another foe could take advantage of the fact the swarm is currently immobile to blast it with an area effect.