How to put a limit on the "Create an Advantage" action?

Question

Well, I'm running a FAE single player campaign and I gave my player 3 companions. He's a wizard of sorts and on his group are a girl who can paint "weathers", a super strong luchador and a girl who uses "pixels" on real life to create some small stuff like shields and potions that fade quickly.

They're gonna face a floating monster the size of a citadel next session, and when he saw it he said: "I know what I'll do... I'll make a chain so big it can reach him and we'll climb thru it!"

Ok, not a bad idea sine well I didn't even really knew how they could do it since to make things more interesting I banned every aspect that lets them fly unless it's scenario-given, he said he would use the Create an Advantage action to do it.

I tought I'd let him but then I realized there's an ally on his party that can do it too, the pixel girl. Then I realized the Create an Advantage situation is kinda broken...

I don't want to reach the point of game-breaking situations, he's my friend but we argue a lot and he's the kind of player that's always thinking on how to "maximize" character potential. We're not on that point yet but I want to avoid those situations...

Answer 1

Your greatest tool here is the ability to set difficulties.

Setting an unusually hard difficulty is good when you want to emphasize that it's not something which can be done easily. The character can still do it, if they roll well or spend resources or both, but they can't do it casually or frequently without running out of resources.

Fate Core talks about what to keep in mind when setting difficulties.

...things should generally be more challenging when the stakes are high and less challenging when they aren’t. (FC 191)

No one will bat an eye at you looking at the relevant situation aspects and giving a +2 to the opposition for each one, because it mirrors the invoke bonus they get. (FC 192)

I'd also like to quote the Dresden Files RPG on setting difficulties for situations like this:

The difficulties for declarations should, honestly, be based on how interesting the proposed fact or aspect is. Ideas that would disrupt the game or are just unreasonable should simply be vetoed. These are the questions to ask yourself when determining difficulty:

1. Is the declaration interesting (or funny)?
2. Will the declaration have interesting consequences if it’s acted upon, whether it’s right or wrong?
3. Does the declaration propose a specific and interesting course of action?

Each “no” adds 2 to the base difficulty of Mediocre. If the proposed fact is very amusing, proposes an interesting course of action, and has interesting consequences (three “yes”-es), a Mediocre difficulty is appropriate—you want to provide a good chance that the detail is true. (Your Story 313)

This isn't an attitude Fate Core explicitly endorses, but it's a good thing to keep in mind.

Aspects can say "no."

One of the purposes of an aspect is to make things true. If there's an aspect in play which implies that the proposed action is impossible, then it's okay to say so.

Don't abuse this, though, because Fate characters are competent and proactive. If you say "no" too much, you're dropping walls in front of the players' fun. Most of the time it's better to set high difficulties instead, so they have obstacles to hurdle rather than walls which stop them.

Answer 2

Create an Advantage is limited by the fiction. If they're all-powerful wizards/pixel-artists, then no, there's no limit.

It sounds like you do think their powers have limits though, but you haven't actually talked together about the story of how their powers work. Do that, and the limits will be obvious.

Basically, the Fate system is not going to do this for you. Fate sits back and doesn't do anything until you ask it to represent something about the fiction. Figure out what the fiction is supposed to be, and then it will be very easy to see what Fate rules to use to make that happen.