You do know how to make a boss fight right?
If not, just give the "boss" an event number, lets say 350, give it the flag 200 (Call event on death). So event 350 will play when it dies.
The trigger event, we will make it a H/V trigger (0046), make it vertical, flag 1000. Event 300. In event 300, just put
<KEY<MM0<WAS<MSGDo I need to choke a bitch?<YNJ0301<CLRoh btw,<WAI0010 you are the bitch.<NOD<CLR<CLO<WAI0100<BSL0350<DNP0300<END
this is just an example, feel free to play around with it and ask me a question or two. For major bosses, its best for you to copy script from the original Cave Story.