How do I make enemies speak in battle?
The simple answer is to create an attack that does nothing. You can set it's caption to some text, and it'll show on the bottom of the screen. Just make it look like the boss is speaking.
You can also attach the text to hits, but that line will be said every time that attack is selected.
If you're looking for a scene, like Kefka's taunting at the end of FF6, the best bet is to create an invisible untargettable monster, and set it's only attack to an attack called Start Taunt. Make Start Taunt chain to a second taunt (at 100%) with a respectable delay, and so forth.
The effect is, every five seconds or so, the 'boss' will appear to say a line during the fight, without actually wasting an action.
For an example, during the Jormungandling fight sequence of Wandering Hamster, where Bob is tied up, he's included as a monster with four speech attacks. The snake has a bite attack, whose caption was Bob screaming that it's biting him. (Incidentally, the attacks are set up so that his regular attacks target an "ally" (Bob in this case), and his Alone attacks target the two cats (when Bob is "dead"))
You can make these as complex as you wish. For example, if you want a boss to say something on death, have him spawn an invisible creature on death whose only attack is a suicide attack with caption "Blaargh! I am dead!"