As with other RPGs, Final Fantasy Legend also has an RNG. It is difficult to manipulate, as it will only increment through actions, or get shuffled by a soft reset. The soft reset RNG manipulation is mostly frame-based, but something like entering a menu can slightly affect it.

Unfortunately, soft resets seem to revert to a default for the battle RNG, meaning if it's impossible to win after a reset, there's no way to manipulate that short of getting into another battle before the impossible one without resetting.

Final Fantasy Legend uses a different address for almost every different effect possible. Manipulating one will not affect whether a different action is successful, as almost every action that needs an RNG has its own address it can use.