Promotions exploit:

In short, once you get one character to lv. 100, you can start freely promoting any characters in the game. I think in a longer speedrun category it would be worth it to even go slightly out of your way to get a lv. 100 character as fast as possible. After all, promotion means upgrade stones, and the related upgrade stone exploit lets you give max stats to characters pretty easily.

"Wallzipping":

This is a glitch that could come in handy situationally. Doing it is pretty inconsistent (except with Zhang He), but it should still help in replacing some infinite jumps in a glitched run for optimization purposes.