[Hishin]

On various websites it says this emulator should be able to play a GBC game like say "Shantae" in its GBA mode for the hidden content.
BUT HOW DO I DO THAT!?
Nobody even shows how its done as if it should be obvious and the emulator keep auto detecting it in GBC mode.

[FAST6191]

In the case of Shantae you might like http://www.romhacking.net/forum/index.php/topic,16827.msg252793.html#msg252793 as it will get the stuff without the downsides.
Not sure what goes for Zelda Oracles and Ages right now, the other two games that have any notable changes for the GBA but there is probably something out there.

To answer the question though it was an option somewhere in the menus, afraid I have not got it on this machine to test. Alternatively if you are using a good GB/GBC emulator (VBA will get it done but there are better options like http://bgb.bircd.org/ ) then there should be a fake GBA option somewhere in there, possibly next to whatever allows you to pretend it is a super gameboy.

[KingMike]

It is right in the Options menu. Gameboy -> GBA. (enable it before running a game, or else reset the game afterwards to apply the setting)

That should do it. I recall the Zelda Oracle games had an exclusive ring near the beginning of the game, but I can't recall offhand where it is to fully text. The colors are kind of washed out, so that's a sign it's running in GBA mode.