Here's what you need to do.Steps to Making the Event
-Make a Switch for the door to open after killing the boss (name it whatever you like, but relevance to what its doing helps)
-Make an Event Page for the the door that looks like this:
Which of course, it doesn't have to be EXACTLY like mine, you can alter the Sound Effects, Door Graphic, and of course, the Transfer Player option would be set to wherever you would like the player to go to.
ALSO: Don't forget to make the switch turn on after you kill the boss, because if you don't, the door will not open for you.
Just try it out, play around with a bit, and let me know if you need further help. Also, if you don't understand how I did something or why, just ask, it doesn't bother me to help people out. We all have to start somewhere.
EDIT: Yeah.. I just re-read your post like 10 times lol, and I saw that you said you wanted the effect of it slowly opening after killing the boss, not really sure how to make that one play out, but this event will do exactly what you want, except that you will have to manually open it instead of it slowly opening by itself.