Advanced -> Intermediate?

Way to bump a discussion from last year without really adding anything to it, but if you really want to continue it...the only real classification that exists in game and is clearly defined is the bolt spells. From there, you have a mix of contradicting terms, translated text, and the comments from other users.

You could just base it off of how you get the skills: you need the previous "tiers" at rF+.

bolt spells: There are no skill prerequisites for learning these skills.

IS, TH and FBall: In order to start any of the 3 skill quests, one prerequisite is that you need the 3 bolt spells at rF.

Hail (and the other 2 possible skills?): This only applies for hail since it's the only one released so far, but you need the previous "tiers" of ice spells at rF in order to start the skill quest. (ignoring ice shield since it's not part of this discussion)

Now throw in the Mabi difficulty ranking (basic, int, adv, HM) and you've got one possibility of how the above skills are classified.