You don;t need anything like that. The OP really wants the FFC to create effects while Link is on screens with specific creatures, not when actually leaving a screen. It's pretty straightforward to make an FFC affect Link while it is true, and as soon as Link leaves the screen, the effect can become un-true.
If you want to go further, you can create global variables, in boolean form, or as ints, and then use the FFCs to change their global value, until something else changes them again. you could, for example, make an FFC that disables certain items, setting them to false.
If you do this, you want to ensure that you create a marker that indicates if they were true before, and that they were set to false by a specific effect. Then, you can have a second FFC that checks to see if that effect set anything to false, and resets it to true; but if it was not set to false by that effect (because your boolean flag isn;t set to true), then it doesn't make it true. (This prevents giving Link items that he didnlt have, if he encounters the screens that set it back to true.)
@UninspiredUsername404: I could probably make this for you pretty easily. If you specify exactly what effects you want, I can work on it sometime in the next few days. What you want is an FFC that changes certain attributes when Link is on the same screen, but when Link leaves the screen, or performs a certain action,. the effect ends. That works best with a while(this->isValid) loop, because you can create an event that removes the FFC within that FFC, so you only need to put one FFC on a screen to achieve the entire effect, and only on screens where that effect should happen. You do not need to put FFCs on adjacent screens to achieve this goal: One FFC can achieve all of what you want to do.
You could tie the FFC to the enemy, or a list of enemies, with if statements; or to other conditions, with if statements. I have some things set up like this in my projects, which are rather close to what you are describing, but I haven;t had the time to finish them, because the major components (the effects that the FFC will trigger) are unfinished.
If the actual effects that you want to cause to happen to Link are complex, it will take longer, but if it;s something simple, like setting items to false, then I can provide you with everything you will need, and of course, you can expand on it later. Send me a PM if you want me to lend you a hand.
If you want to tie the effect to an enemy being on the screen, I can easily ensure that the enemy is set in the FFC arguments, so that you can recycle it for multiple enemies, with different effects. I can further make the actual effects selectable with arguments, reducing your code overhead to one script.
Otherwise, if you want to handle this on your own, that's fine too.