This is the first time anyone's really ripped into my game, so I appreciate the feedback!
The teaching skills mechanic was a risk for sure, but I liked it a lot going into release. As far as enjoyability goes, I see the whole mentor system as having one character in your party plus an extra turn to learn skills, buff stats, or heal. Had I nerfed the encounter scaling and rates, I feel like this would have come across better. While I could have easily had skills be permanent, I thought this would make Reid useless after a while. However, Reid does learn some valuable skills which he could use turn 1 instead of teaching forgetful Harold a skill he already learned. (Speaking of which, guaranteed escape when under half health is one of these skills! Reid learns it at level 3.)
I also should've used a region spawn to have Unga Bunga only spawn near the cliffs in the main overworld. Since you'll probably be higher level by the time you head for the cliffs, it makes sense to have the stronger enemies spawn there. During one test run, I had a nasty encounter with an Unga Bunga at level 1 and got away with <15 hp on both characters. Unga Bungas are pretty much guaranteed to be slower than you so it's at least easy to get away from them.
Flame Slash does have its advantages, although I didn't make all of them super clear. It deals magic damage for less MP than Spark and does 150% damage against enemies like Maneaters. What I wish I did was had a log for damage rates so you could easily learn which attacks enemies are weak to or resistant against. I also originally had it burn the opponent, but the burn state was a poison clone which didn't work in battle and I ended up scrapping it. Perhaps I could bring that back to make it a bit more unique.
Again, I appreciate the feedback. It was definitely a squeeze just releasing Stuck In The Past as is, but having a little more RPG smarts could have definitely helped me go above and beyond with it.