For when rather than fight one monster, you want to fight all of them.
A phasm's base chassis is underwhelming, to say the least. 15 aberration HD, medium size, no stat boosts higher than +6, an insultingly bad natural attack, and speed equal to a human's.
The various racial traits help somewhat (Amorphous gives nice immunities, Resilient compensates for your weaker saves, and telepathy is telepathy, but the phasm would still be an easy -0 if it weren't for one single ability: Alternate Form.
The phasm's version of it allows it to turn into any creature as a standard action, as long as it's large or smaller. I honestly can't comprehend how something that can turn into an Infernal is a CR 7 monster, but apparently it is.
Finding any kind of comparison at all was hard enough, but in the end I settled on the Master of Many Forms. Barring gruesome abuse such as the above example, a 10th-level MoMF and phasm are both ECL 15 shapeshifters, with the MoMF being able to turn into larger creatures (including useful forms such as the hydra and kuo-toa leviathan) and getting extraordinary special qualities in addition to whatever benefit the first five levels of its build gave, while the phasm is unconstrained by type or HD amount.
Are the two completely equal in power? Definitely not: the difference in assumable forms will undoubtedly work in the favor of one or the other (probably the phasm). That said, I feel like the phasm and MoMF are on the same general power level: both are broken if abused and strong tier 1-2 characters if not.
With phasms being so similar to the (not necessarily unbalanced) MoMFs, I'll give them +0 LA.
Next are planetouched.