Shimmerling swarms are 11 RHD Fine-sized swarms of fey. Their ability scores are okay-ish, with predictably low strength (the idea of 1000 cubic foot of hiveminded creatures not being able to cooperate to lift a few pounds has always bugged me, but that's just how the game is), high dexterity, reasonably high wisdom and charisma and lowish constitution and intelligence.
Being Fine swarms, they obviously have the Distraction ability, in addition to being immune to weapon damage and single-target spell effects. Shimmerling swarms have a hivemind, which sets their intelligence to 1 once the swarm's health drops below 1 HP/HD.
A shimmerling swarm's main form of 'attack' is Dazzling Illumination, which has various effects. Firstly, it creates an area of Daylight centered on the swarm. Secondly, it dazzles all sighted creatures in the area (yes, that does happen to include allies). Thirdly, it might Fascinate creatures that see it.
The issue with the last bit is that it's a non-friendly 300 ft-range effect that's nearly useless in-combat because Fascination gets broken by obvious threats. Arguably, a shimmerling swarm is better switching it off entirely: at least that way the party won't be stopping to stare at the pretty lights every minute.
The combination of a huge pile of RHD, bad abilities, no easy advancement and all the inherent difficulties of being a swarm mean that the shimmerling swarm is a relatively easy -0 LA. Just play a low-HD swarm with class levels to basically outclass this in every way.