Not sure what you mean by trigger. How I do bosses:
Set an event, choose a graphic (usually, depends) alternatively you can make it a cut scene before boss type of fight, up to you really. In that event set a conditional branch - set it as local switch A is off (execute custom handler on or off, doesn't matter for a boss I don't think).
Now this boss is blocking the way so you have to encounter him. So I've set him as Hero Touch. When you touch him the local switch A will be off and when the switch is off make sure the following happens:
Change Battle BGM for the boss music
Then Enemy Encounter for the boss (custom defeat handler I have on)
Now when you defeat the boss, make sure that:
Change Battle BGM for the normal battle music
Local Switch A = ON
Create a new page. On this page have the preconditions as "Local Switch A = ON", and have Local Switch A = ON as the only thing in the event list for this second page (has to be the second page I think). Have the graphic as nothing, and the event as parallel process. Now when you leave the map and return the game remembers that for this event Local Switch A = ON. Same way to create treasure chests that don't reappear. Local Switches are sex.
Hope that hopes.
EDIT: Friggin' hell.