The fanservice in Shakugan no Shana is generally tame and not really worth discussing. That is unless you are talking about the two scenes where they actually show Shana’s tiny little boobs. Uncovered and uncensored.
Granted, this nudity is about as subtle as you will ever see, but all things considered, it’s still pretty mind-blowing.
Shakugan no Shana (manga and light novel, complete)
This is the only illustrated nudity in the entire manga and light novel series. The first nude scene occurs in the 13th light novel, and lasts for three total images. The second scene is in the 8th manga volume, which goes into slightly more detail, though still attempts to be subtle about it.
There is a decent amount of fanservice throughout both, but other than these two scenes the service level always remained mild, and wasn’t really worth including.