Let's say a PC plants a proximity mine or magical rune or suicidal summon somewhere in a room he would expect some enemies to walk in later on, and then leaves to take care of something else or simply lies in waiting. The total roll for it is, let's say, a +4.
When an enemy walks in, is it a good idea to model it as a delayed Attack action that provides a passive value improvable by invocations, or should I do something else? I feel that Create an Advantage wouldn't exactly cut it here...
You use The Bronze Rule, AKA the Fate Fractal, and treat those environmental factors/Aspects as characters, allowing them to make an attack (emphasis mine):
In Fate, you can treat anything in the game world like it’s a character. Anything can have aspects, skills, stunts, stress tracks, and consequences if you need it to.
This is called this the Bronze Rule, but you may also have heard of it as the Fate Fractal if you pay attention to the Internet. You’ve already seen some examples of this in other places on the site; you give your game its own aspects during creation, you place situation aspects on the environment as well as on characters, and the GM can let environmental hazards attack as if they had skills.
So, you roll whatever you'd roll to set the trap, which would be Create An Advantage to place the aspect "explosive runes". The bad guy walks in, and the GM treats the runes as a character, allowing them to roll an attack against the bad guy. What level should the runes' skill be at? That's up to you and the GM, although a quick-and-dirty way to decide is to simply set their Attack rating to whatever you rolled. So if you rolled a +4 to set the trap, it'd attack at +4.
Simple: The PC rolls an attack action when the device activates. The planting is just a story detail that justifies an attack by an absent PC.
Of course it is a good idea to create an advantage when planting, so that it can be invoked during the attack, or compelled to complicate matters.
I point you to the Golden Rule of Fate:
Decide what you’re trying to accomplish first, then consult the rules to help you do it.
What would it look like in-fiction? Is it a preemptive strike, designed to cause confusion right before the battle? That's creating an advantage.
Do you want it to be an attack that does stress? Then treat it as an attack. And there is no need to make the roll prior, you roll the attack when it happens. Of course, you might want to create an advantage anyway to make sure the mine/rune/etc is properly hidden. In which case it not only gives you an aspect to invoke, but also gives narrative permission to attack with Crafts.
While the Golden Rule of Fate mentioned above helps,you also need to decide how important that setup is. Because of the action economy in FATE, you should be very careful about applying the fractal to things the PCs do. Otherwise, you risk giving one PC the effect of multiple actions.
With a few possible exceptions the words "automated" and "environmental" would suggest that the attacks are secondary to the other action occurring, and so you shouldn't spend much time on them because they're not the meat of that scene. So let the PC do Create an Advantage and have the free invokes off of it.
If for some reason the automated attack is crucial to the scene, then fractal it.
If it's somewhere in the middle, then you could go with an attack action.
