I'm trying to make it so that some trees in a forest disappear after the elite 4 are defeated, leading to a postgame Legendary encounter within the same map. Any ideas how to do this? I tried overlaying an event onto the tree tiles which doesn't seem to work, and I know I could make a near-duplicate map that the player is transferred to after the switch is triggered but I'd need to make individual switches for all the items and trainers in the map as well which I'd prefer to avoid.
Instead of overlaying events on top of the tree tiles, try making those events have the tree tiles set as their graphics, and remove the regular tree tiles that are part of the hidden path. When you select a graphic for an event, the second option is called "(Tileset)", so you can change its graphic there. Then, make all the events have a blank second page that is activated by some switch.