I think a better way to go about single player training modes (which I think would be a great thing) would be to scrap the whole "alone, invincible" thing. Give the player NPC allies with some modicum of intelligence about what to do in the fight, give the player normal stats, and let them go wild. There is no mechanic in this game that an NPC couldn't be programmed to handle with at least some effectiveness, and giving the NPC's a larger HP pool to both allow for their AI shortcomings and allow the player to actually see what is happening for any length of time would be perfect for training. For particularly difficult or obtuse mechanics, the NPC's can even throw out hints like "you better not point that thing at me!" when you get shrieked in T7 or "pass me that rot!" when you get Allagan Rot in T2, etc. If you die repeatedly, the game starts giving out hints on some of the general fundamentals of the fight, like how dying repeatedly at the final boss in Dissidia prompts the game to give you some tips on the fight.

These fights would be every bit as unforgiving as their multiplayer counterparts, but without the headache of finding players who are actually willing to help a new guy, dealing with players when you/they wipe, and the whole no-spoiler experience. You could go into them in any role, and the NPC's you're given and the advice they give would change accordingly. These modes can even count towards story completion for those who only care about that (it seems they're heading this way for low-level content anyways, from what the developers of said). I for one would absolutely sit down and master every aspect of a fight in every role in these training modes before I ever step foot into the fight with others if the game let me. Screwing over other people (friends or strangers) with my own inexperience has never been something I considered fun.