Serialize selected fields
Depends on why you think you need fields. Fields in Drupal come with their own structure, they are exposed to Views, etc. Lets assume you manage to store a number of fields in a single serialized blob. How would sorting, filtering work?

Perhaps what you are looking for is more along the lines of a custom field widget, that exposes a large number of textfield forms, and stores the data captured in a serialized entry as opposed to 350 individual fields?