I know swarms are immune or resistant to weapon damage depending on size, and can't be grappled, tripped bull rushed a such.
Are they still subject to spell effects, like the save or stagger from the Snowball spell? I will be playing a Witch soon, and at low to medium levels I need to make sure i'm ready to fight swarms, possibly with spells other than Burning hands.
Keep in mind that the snowball spell in particular is single target, which means it will not effect a swarm.
Well lets look at the swarm traits.
A swarm has no clear front or back and no discernible anatomy, so it is not subject to critical hits or flanking. A swarm made up of Tiny creatures takes half damage from slashing and piercing weapons. A swarm composed of Fine or Diminutive creatures is immune to all weapon damage. Reducing a swarm to 0 hit points or less causes it to break up, though damage taken until that point does not degrade its ability to attack or resist attack. Swarms are never staggered or reduced to a dying state by damage. Also, they cannot be tripped, grappled, or bull rushed, and they cannot grapple an opponent.
A swarm is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate), with the exception of mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms) if the swarm has an Intelligence score and a hive mind. A swarm takes half again as much damage (+50%) from spells or effects that affect an area, such as splash weapons and many evocation spells.
Swarms made up of Diminutive or Fine creatures are susceptible to high winds, such as those created by a gust of wind spell. For purposes of determining the effects of wind on a swarm, treat the swarm as a creature of the same size as its constituent creatures. A swarm rendered unconscious by means of nonlethal damage becomes disorganized and dispersed, and does not reform until its hit points exceed its nonlethal damage.
Emphasis mine.
So no, swarms cannot be staggered. There is also a great deal of other things they are immune to listed here. However, any condition not listed here they are susceptible to.
The other part that deserves special mention is "A swarm is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate)". So many spells that apply a condition don't apply to swarms, either.
Rereading the description, I actually think that the swarm is immune not do to that line, but due to "A swarm is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate)". The description of the sowball spell specifically says single-target.
I read your bolded section differently. Not that swarms are immune to stagger, but that they are immune to the staggered conndition when at 0HP like most other creatures.
That actually is a fair point, it simply means they are not staggered by HP damage.
Try something like frost fall.
