Super Mario Bros. X Forums

You need 3 layers: block 1, block 2 and block 3.
You need also 7 events: 3 for show the blocks,d 3 for the delay of the show of each blocks and one for start all.
The events start when you arrive at the boom-boom, so take the boom-boom, go to events and put in activate the first event.

In addition to what Christian07 said, you'll want to use a "set position" event. Edit the section borders so that the boss arena is the only part of the room that's actually in the section. Edit the event that seeing the Boom Boom activates, and under "set position" select "use current". Now change the section borders back to normal. This way, when the player sees Boom Boom, the screen will scroll into the boss arena and the rest of the room will be inaccessible. Along with what Christian07 said, the combined events should be almost identical to the transition from SMA4.