A perfect game without using glitches would probably be incredibly tedious, I'd have to look into it a bit more. I suppose gaining Gil in this game is ridiculously easy, but there are other annoying items to get that would drive me crazy. Definitely something to think about though.
Also,
Reserve characters would gain 1/2 EXP, while Aeris's level increases would affect the levels of every character that joined after her.
Reserve characters gain experience when they're KO'd? As I wrote that previous post I realised I can't remember if you can switch them out or not when they're KO'd. My idea was to kill all the party members (including the reserves) and just have her casting Matra Magic on the groups of enemies in the Mythril Mines. It doesn't stop her from levelling up, but all the character's starting levels are determined by the party average + or - a specific number. This way you'd keep the party average as low as possible.
Of course, to keep it the lowest possible a better idea would be to kill Cloud as soon as you're about to leave sector 5 for Wall Market, then kill 160 enemies with Aerith (80 on Level 1 then 80 on Level 2). If you're going to use this approach to keep the level gains as minimal as possible, it might be better to wait until you get the striking staff in the Train Graveyard (then either kill the enemies there or head back into the last screen of the sewers where they give less EXP).
Thinking about it, you'd make enough Gil from killing the enemies with magic in sector 5 to buy Tents to replenish MP. So yeah you could do this too. The less tedious approach is definitely the enemies in the Mythril mines though, it takes about 2 hours to get every character to their level 3 limits that way, and you're only doing one.
EDIT: I'd run away from hell house at first. I can't imagine he's a lot of fun to beat with Aerith at that low a level, considering he can give you a game over with a full party when you're trying to run away.
